Display apparatus and method

ABSTRACT

A display apparatus includes a spatial light modulator, such as a digital micromirror device or an LCD panel, and a lighting unit that illuminates the spatial light modulator. The lighting unit may include red, green, and blue lamps which emit light that impinges on a DMD from at least two different directions. The lamps may be flash tubes which are fired at different binary levels in accordance with the rank of the bits that are being displayed on the DMD. The lamps may be fluorescent lamps which are driven steadily at predetermined levels while the rows of micromirrors are turned on in sequence and subsequently turned off in sequence. Resetting to dislodge micromirrors that have become stuck can be accomplished by emitting current pulses through the micromirrors while exposing them to a magnetic field. The illumination unit may include a lamp unit and a color wheel. The light from the lamp unit can be integrated, and the data displayed on the spatial light modulator can be changed when the integrated light reaches a predetermined value. The color wheel may be rotated faster than the frame repetition rate of video information that is being displayed. The intensity of the light may be controlled in accordance with the bit rank or significance of the bits that are being displayed by the spatial light modulator. Several techniques for achieving different intensity levels are disclosed.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present patent application is a continuation-in-part of Applicant'sapplication Ser. No. 08/381,156, filed Jan. 31, 1995 now U.S. Pat. No.6,348,907, and a continuation-in-part of Applicant's application Ser.No. 09/063,364, filed Apr. 21, 1998 now U.S. Pat. No. 6,535,187. Thedisclosures of both of these parent applications are incorporated hereinby reference.

It is noted that application Ser. No. 08/381,156 was filed during thependency of Applicant's earlier application Ser. No. 08/034,694, whichwas filed on Mar. 19, 1993. application Ser. No. 08/034,694 was filedduring the pendency of Applicant's earlier application Ser. No.07/862,313, which was filed on Apr. 2, 1992. Application Ser. No.07/862,313 was filed during the pendency of Applicant's earlierapplication Ser. No. 07/521,399, which was filed on May 10, 1990.Application Ser. No. 07/521,399) was filed during the pendency ofApplicant's earlier application Ser. No. 07/396,916, which was filed onAug. 22, 1989. The disclosures of all of these prior applications arealso incorporated herein by reference.

Application Ser. No. 07/521,399 matured into U.S. Pat. No. 5,128,782,which issued on Jul. 7, 1992, and application Ser. No. 08/034,694matured into U.S. Pat. No. 5,416,496, which issued on May 16, 1995.Application Ser. No. 07/396,916 and application Ser. No. 07/862,313 havebeen abandoned.

Although at the time of filing the present application, Applicant doesnot claim the benefit under 35 U.S.C. § 120 of application Ser. Nos.08/034,694, 07/862,313, 07/521,399, or 07/396,916, Applicant reservesthe right to claim such benefit if, at any time during the pendency ofthe present application at the Patent and Trademark Office orthereafter, prior art turns up which makes such a claim for the benefitof an earlier prior date desirable. At the time of filing the presentapplication, only the benefit under 35 U.S.C. § 120 of the first twoapplications identified above (i.e., Ser. Nos. 08/381,156 and09/063,364) is being claimed.

BACKGROUND OF THE INVENTION

The present invention is directed to a display apparatus which employs aspatial light modulator, such as a liquid crystal display or digitalmicromirror device, and to a display method.

A digital micromirror device is a spatial light modulator which employsan array of tiny mirrors, or micromirrors, whose positions can beelectrically controlled in order to display an image. This technologyhas been developed extensively by Larry J. Hornbeck and others at TexasInstruments, Inc. of Dallas, Tex., and is described by them in asequence of patents going back more than a decade. These developmentalefforts have culminated in a digital micromirror device which includesan array of memory cells and a corresponding array of pivotablemicromirrors whose positions are electrostatically adjusted by thecontents of the memory cells. As is perhaps best described in U.S. Pat.No. 5,096,279 to Hornbeck et al, the array of pivotable micromirrorsthat cooperates with the memory cells can be made using integratedcircuit fabrication techniques.

As is described in the above-identified patent, in U.S. Pat. No.5,280,277 to Hornbeck, and in an article entitled “Mirrors on a Chip”that was published in the November, 1993 issue of IEEE Spectrum at pages27–3 1 by Jack M. Younse, a negative biasing voltage is selectivelyapplied to the micromirrors and to landing electrodes fabricated beneaththem in order to obtain bistable operation of the micromirrors andsimultaneous updating of the entire array of micromirrors. Sometimes themicromirrors get stuck. It is known that this problem can be cured bysubjecting the micromirrors to resonant reset pulses whichelectrostatically dislodge any stuck micromirrors.

It is also known to make a color display using a single digitalmicromirror device by sequentially exposing it to red, green, and bluelight impinging from a single direction. A white lamp and a color wheelcan be employed for this purpose. In situations where it is economicallyfeasible to devote three digital micromirror devices to a display, eachof them can be illuminated by light of a different primary color and theresulting red, green, and blue images can then be superimposed on ascreen.

Advances have also been made in other types of display apparatuses. Forexample U.S. Pat. No. 5,122,791 to David J. Gibbons et al discloses aferroelectric LCD panel which is selectively backlit by red, green, andblue fluorescent tubes. The intensity or duration of the backlighting iscontrolled on the basis of the rank of the bits that are being displayedon the panel.

Applicant's U.S. Pat. No. 5,416,496 also employs a ferroelectric LCDthat is back-lit with colored lights. The colored light may be generatedin flashes whose intensity is controlled on the basis of the rank of thevideo information bits that are being displayed. Alternatively, insteadof flashes of light, the LCD panel may be illuminated by light that isgenerated steadily, and whose intensity is determined by the rank of thebits that are being displayed. In the latter alternative, the pixels ofthe panel are turned on in accordance with the video information on arow-by-row basis, and are subsequently turned off in accordance with thesame video information, again on a row-by-row basis. As a result, eachpixel that is turned on and then turned off receives the same amount oflight regardless of its row, so the LCD can be addressed row-by-row withvideo information while the LCD is being illuminated.

SUMMARY OF THE INVENTION

A primary object of the invention is to provide an improved displayapparatus which employs only one digital micromirror device.

Another object of the invention is to provide a display apparatus inwhich a digital micromrirror device is exposed to light at differentbinary levels.

Yet another object of the invention is to provide a display apparatuswhich employs a digital micromirror device wherein the micromirrors arenot updated all at once, but are instead updated on a row-by-row basiswhile being exposed steadily to light.

Another object of the invention is to provide a display apparatus whichemploys an addressable spatial light modulator that is illuminated by alighting unit whose light output varies in intensity in accordance withthe bit rank of video information that is being used to address to thespatial light modulator, with the light output of the lighting unitbeing monitored in order to determine when to change what is displayedon the spatial light modulator. The video information may be fed to thespatial light modulator on a frame-to-frame basis for each color, or ona row-by-row basis for each color. If the video information is fed tothe spatial light modulator on a row-by-row basis, the amount of lightreceived by different rows can be equalized, during display of aparticular bit rank of video information for a particular color, byturning the pixels on row-by-row in accordance with the same videoinformation.

Another object of the invention is to provide a display apparatus whichemploys a spatial light modulator that is illuminated by a lamp unithaving a plurality of lamps, with the light intensity being adjusted byturning at least one of the lamps on and off.

Another object is to provide a spatial light modulator that isilluminated by a lamp unit having a single lamp that is driven atdifferent intensities, depending on the bit rank that is beingdisplayed. Instead of a single lamp, a plurality of lamps that aredriven in unison may be used. For example, a plurality of lamps may beconnected in parallel to supply more light than could be delivered by asingle lamp.

A further object of the invention is to provide a spatial lightmodulator that is illuminated by a lamp unit which emits light with anintensity that is constant, with the intensity being controlled beforethe light impinges on the spatial light modulator (or after impingementon the spatial light modulator, if preferred) by passing the lightthrough at least one attenuator. The at least one attenuator may be aplurality of rotating attenuators, possibly combined with a color wheel.Alternatively, the at least one attenuator may be a liquid crystal panelhaving rows that are selectively turned on in accordance with thedesired light intensity, or a liquid crystal cell which is pulse-widthmodulated in accordance with the desired intensity.

A further object of the invention is to provide novel techniques forilluminating a spatial light modulator through a rotating color wheel.If the color wheel is rotated more than one revolution during display ofa frame of video information, different bit ranks of the videoinformation can be allocated to different revolutions. Furthermore, themost significant bits can be partially displayed during one revolutionand subsequently completed during one or more additional revolutions.

A still further object of the invention is to integrate the lightemitted by a lighting unit whose intensity is changed through aplurality of levels in order to control the duration of buffer periodswhich accommodate relatively slow changes in the light intensity orerratic light output during transitions from one level to another, thebuffer periods being periods when the data displayed on the spatiallight modulator is such that all of the pixels of the spatial lightmodulator are turned off. The buffer periods may have durations that arecontrolled by monitoring the light generated by the lighting unit. Thebuffer periods may also have fixed durations, corresponding in durationto the time needed for a color wheel to rotate completely through one ormore colored sectors or through one or more complete revolutions.

In accordance with one aspect of the invention, a display apparatusincludes a digital micromirror device having an array of movablemicromirrors, along with exposing means for exposing them to light of afirst primary color which impinges on the array from a first directionand to light of a second primary color which impinges on the array froma second direction. The first and second directions may lie in a commonplane, which permits a micromirror that is ON with respect to the firstprimary color and OFF with respect to the second primary color when it(the micromirror) is in one of two positions to be OFF with respect tothe first primary color and ON with respect to the second primary colorwhen it (the micromirror) is in the other of the two positions. In thissituation the light of the first and second primary colors impinges onthe array at different times, possibly in sequences of flashes havingdifferent binary levels.

In accordance with a further aspect of the invention, a method fordisplaying a sequence of frames of video information on a digitalmicromirror device is provided. The digital micromirror device has anarray of micromirrors that are disposed in rows and that are movablebetween a first position and a second position. The video informationfor a frame includes a plurality of first multi-bit video words (such asmulti-bit video words for the red component of an image), and eachmicromirror corresponds to one of the first multi-bit video words.Furthermore, each of the first multi-bit video words includes at least amost significant bit and a least significant bit. The displaying methodincludes the step of moving micromirrors which correspond to first videowords whose least significant bit has a predetermined value from theirfirst positions to their second positions, the micromirrors of a firstone of the rows being moved before the micromirrors of a last one of therows. This is followed by the step of returning the micromirrors thatwere moved during the first step to their first positions, themicromirrors of the first row being returned before the micromirrors ofthe last row. The display method also includes the step of steadilyexposing the micromirrors to light at a first level while the first stepis conducted and while the second step is conducted. If the micromirrorsare activated on a row-by-row basis when the first and second steps areperformed, and if the first and second steps are conducted at the samerate, each micromirror that is moved from its first position to itssecond position and then back to its first position receives the sameamount of light while in the second position, regardless of themicromirror's row. Consequently the micromirrors need not all be movedat once despite the steady illumination, It is noted that themicromirrors need not all be moved at once if they are illuminated indiscrete flashes, either, instead of by steady exposure in accordancewith this aspect of the invention.

In accordance with another aspect of the invention, a method for using aspatial light modulator can be conducted by displaying data on thespatial light modulator, shining light on the spatial light modulator,integrating the light, and changing the data displayed on the spatiallight modulator when the integrated light reaches a predetermined value.The method may further include changing the intensity of the lightshined on the spatial light modulator, either by using a lighting unithaving a plurality of lamps and turning at least one of the lamps on andoff, or by using a lighting unit having a single lamp that is driven atdifferent energy levels during a sequence of time periods. This latteralternative may be modified by driving a plurality of lamps, in unison,at different energy levels during the sequence of time periods.

A color wheel may be used to color the light, preferably (but notnecessarily) before it impinges on the spatial light modulator. Thecolor wheel may be rotated at a rate faster than the frame repetitionrate. This can lead to several advantages. One is that some of the bitranks for all three primary colors can be displayed during onerevolution of the color wheel, and other bit ranks can be displayedduring one or more subsequent revolutions. Buffer periods can be used toadjust the amount of illumination received by the spatial lightmodulator in accordance with the bit ranks. Another advantage is thatthe display of the most significant bits for a frame may be spread overtwo, and possibly more, revolutions of the color wheel. This means thatthe total amount of light of a particular color that impinges on thespatial light modulator is not limited by the product of the lightintensity and the time needed for the color wheel to rotate through asingle colored sector. For example, the spatial light modulator may beilluminated with red light during display of the most significant bitsof the red component of an image for a period corresponding to therotation of the color wheel through an angle of 200_, with half of thisangle plus a buffer period occurring during one revolution, and theother half plus another buffer period occurring during anotherrevolution. Illumination for the green and blue components can, ofcourse, also be conducted in this manner. A further advantage is thatbuffer periods, when all of the pixels are off, may be inserted duringrotation of the color wheel through one or more colored sectors orthrough one or more complete rotations to absorb slow or turbulenttransitions from one light-intensity level to another.

According to a related aspect of the invention, a method for using aspatial light modulator can be conducted by displaying data on thespatial light modulator, shining light on the spatial light modulator,coloring the light with a color wheel (preferably before the lightimpinges on the spatial light modulator, but possibly after impingementof the light instead), and rotating the color wheel faster than theframe repetition rate. The method may further include integrating thelight and changing at least some of the data displayed on the spatiallight modulator when the integrated light reaches a predetermined value.The most significant bits for all three primary colors may be displayedduring two or more revolutions of the color wheel, and different bitranks for all three primary colors may be displayed during differentrevolutions. Furthermore, the intensity of the light shined on thespatial light modulator may be changed as the color wheel is rotated.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a display apparatus in accordance with afirst embodiment of the present invention;

FIG. 2 is a top plane view of a detail marked 2 in FIG. 1, and shows aportion of an array of micromirrors;

FIG. 3 is a cross-sectional view taken along line 3—3 of FIG. 2;

FIG. 4 schematically illustrates a lighting arrangement which is used inthe first embodiment to expose the digital micromirror device to lightimpinging from two different directions;

FIG. 5 is a flow chart illustrating part of the operation of the firstembodiment;

FIG. 6 is a block diagram illustrating an illumination unit in a secondembodiment of the invention;

FIG. 7 is a block diagram illustrating a lamp driver unit in a thirdembodiment;

FIG. 8 is a block diagram of a display apparatus in accordance with afourth embodiment of the present invention;

FIG. 9 is a top plan view schematically illustrating some of themicromirrors in the array shown in FIG. 8 and a reset circuit that emitspulses of current which interact with a magnetic field in order todislodge any micromirrors that become stuck;

FIGS. 10A, 10B, and 10C pertain to the fourth embodiment andrespectively illustrate the rank of bits that are displayed, how thewriting, erasing, and maintaining of information displayed on thedigital micromirror device depends upon the bit rank, and how theintensity of the lighting depends upon the bit rank;

FIG. 11 illustrates a portion of the micromirror array in a sixthembodiment, which employs light impinging from three differentdirections;

FIG. 12 is a block diagram illustrating the construction of a displayapparatus that can be used to carry out a first embodiment of the methodof the present invention;

FIG. 13 illustrates a color wheel that is employed in the arrangement ofFIG. 12;

FIGS. 14A and 14B are flow charts for operation of the arrangement shownin FIG. 12 in accordance with the seventh embodiment;

FIG. 15 is a graph showing an example of changing light intensities inthe seventh embodiment;

FIG. 16 illustrates a flow chart for operating the display apparatusshown in FIG. 12 in accordance with an eighth embodiment;

FIGS. 17A–17N schematically illustrate different bit ranks and bufferregions with respect to the color wheel while two full frames aredisplayed in accordance with the eighth embodiment during fourteenrevolutions of the color wheel;

FIGS. 18A–18C are flow charts which illustrate three of the steps inFIG. 16 in more detail;

FIG. 19 illustrates a color wheel combined with attenuation regions toreduce the light intensity during display of the lower-order bits;

FIG. 20 is a block diagram of a display apparatus in which the spatiallight modulator is a ferroelectric LCD which is addressed with videoinformation on a row-by-row basis;

FIG. 21 illustrates turn-on periods, turn-off periods, and dwell periodsfor different bit ranks and light intensity levels;

FIG. 22A illustrates a flow chart for operation of the arrangement shownin FIG. 11;

FIG. 22B is a flow chart illustrating one of the steps in FIG. 13A inmore detail; and

FIG. 23 illustrates a lighting unit in which the lamp unit has only onelamp, rather than two lamps as in FIG. 12.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Various embodiments of a display apparatus in accordance with thepresent invention will now be described in detail with reference to theaccompanying drawings.

The First Embodiment

With initial reference to FIG. 1, a display apparatus 20 in accordancewith the first embodiment includes an input unit 22 having an inputterminal 24 for receiving a digitalized signal for the red component ofan image, an input terminal 26 for receiving a digitalized signal forthe green component, an input terminal 28 for receiving a digitalizedsignal for the blue component, and an input terminal 30 for receivingsynchronization signals. The digitalized signals for the red, green, andblue components consist of multi-bit video data words (hereafter usuallyreferred to as “video words”), each specifying one of a plurality ofbinary levels for the apparent red, green, or blue brightness ofcorresponding pixels that are to be displayed. The video words for thered, green, and blue components are stored in respective frame memories32, 34, and 36 under the control of a control unit 38. When a full frameis stored, control unit 38 transfers the contents of memories 32–36 tofurther frame memories 40, 42, and 44 and then begins storing the nextframe in memories 32–36. Control unit 38 also reads out the contents ofmemories 40–44 to a digital micromirror device 46 (hereafteroccasionally referred to as “DMD 46”).

DMD 46 is basically an integrated circuit memory having an array ofstatic random access memory cells, addressing means for storing data inthe cells, and tiny movable mirrors or micromirrors which cooperate withthe memory cells. It will be described in more detail with reference toFIGS. 1–3.

The addressing means of DMD 46 includes a serial/parallel converter andregister 48 which receives a series of bits as input data and adjuststhe voltages on column electrodes 50 in accordance with the input data.A gate decoder 52 strobes row electrodes 54 in sequence. Each time a rowelectrode is strobed the data on the column electrodes 50 is stored in arow of static memory cells corresponding to the row electrode. Amicromirror 56 is disposed above each memory cell. The memory cells andmicromirrors together form an array which is designated by referencenumber 58 in FIG. 1.

Each micromirror 56 is supported between a pair of posts 60 by torsionhinges 62. The posts 60 extend upward from a silicon dioxide layer 64that has been deposited on a substrate 66. Each post 60 includesportions of an insulating spacer layer 68, a first metal layer 70, and asecond metal layer 72. A micromirror 56 includes portions of both metallayers, while the torsion hinges 62 are fabricated from first metallayer 70 alone.

Landing electrodes 74 and 76 and actuation electrodes 78 and 80 aredisposed below the micromirror 56. A negative bias voltage isselectively applied to the landing electrodes 74 and 76 and to themicromirrors 56.

The activation electrodes 78 and 80 are connected to complementaryoutputs of a static memory cell 82. When a value is stored in memorycell 82, one of the actuation electrodes 78 and 80 is at groundpotential and the other has a positive potential. This creates a torqueurging the micromirror 56 to rotate clockwise or counter-clockwise aboutan axis 84. Axis 84 is perpendicular to the drawing in FIG. 3 at aposition marked by an arrow 86, which can be viewed as a pivot point.However the magnitude of the bias voltage applied to the micromirrors 56and to the landing electrodes 74 and 76 is selected so that themicromirrors 56 are bistable in their operation. The bias voltageprevents the micromirrors 56 from moving in response to the torqueexerted by the potentials on the actuation electrodes 78 and 80 untilthe bias voltage is relieved, whereupon the micromirrors 56 rotate totheir new positions (if they are different from the old positions) orremain in their old positions (if they are the same as the newpositions), and then the bias voltage is reapplied in order toelectromechanically latch the micromirrors. This movement is indicatedschematically in FIG. 3 by arrow 88. The micromirrors may occasionallystick in one position or the other, possibly due to cold welding to oneof the landing electrodes 74 or 76. Stuck micromirrors 56 can bedislodged by applying resonant reset pulses to the landing electrodesand micromirrors at a frequency corresponding to the resonance frequencyof the micromirrors.

Further details of the fabrication and operation of DMD 46 can beobtained 5 from U.S. Pat. Nos. 5,096,279 and 5,280,277, and from anarticle by Jack M. Younse entitled “Mirrors on a Chip,” published atpages 27–31 of the November, 1993 issue of IEEE Spectrum.

In FIG. 4, a solid line 90 is used to schematically illustrate amicromirror 56 in a first position and a dotted line 92 is used toillustrate it in its second position. In its first position themicromirror 56 is rotated 10 clockwise from a horizontal plane 94 and inits second position the micromirror 56 is rotated about 10counterclockwise. The plane 94 is parallel to the top surface ofsubstrate 66 (see FIG. 3).

With continuing reference to FIG. 4, a lamp such as a flash tube 96 isdisposed in a reflector 98 behind a red filter 100 and a lamp such as aflash tube 102 is disposed in a reflector 104 behind a green filter 106.Flash tube 96 will occasionally be referred to as the “red flash tube”hereafter, and similarly, flash tube 102 will hereafter occasionally bereferred to as the “green flash tube.” Light from flash tube 96 isdirected through a collimating system 108 to a half-silvered mirror 110.Mirror 110 reflects half the light from flash tube 96 to an absorber 112(black velvet, for example) and transmits the other half. A collimatingsystem 114 is disposed between flash tube 102 and half-silvered mirror110, which transmits half of the light from flash tube 102 to absorber112 and reflects the other half. The transmitted light from flash tube96 and the reflected light from flash tube 102 impinges on DMD 46 from afirst direction that is indicated by arrow 116. Arrow 116 is disposed atan angle of 70 with respect to plane 94. When micromirror 56 is in itsfirst position as indicated by solid line 90, light impinging from thefirst direction is reflected at an angle of 90° with respect to plane94, as indicated by arrow 118. This light passes through a projectionsystem 120 to a screen 122. However if light impinging from the firstdirection strikes a micromirror 56 which is in its second position, asindicated by dotted line 92, the impinging light is reflected in adirection marked by arrow 124 to an absorber 126. Arrow 126 is disposedat an angle of 50 with respect to plane 94.

A lamp, such as a flash tube 128, is disposed in a reflector 130 behinda blue filter 132. A collimating system 134 directs light from flashtube 128 (occasionally referred to as the “blue flash tube” hereafter)toward DMD 46, the light impinging in a second direction marked by arrow136. Second direction 136 is disposed at an angle of 70 with respect toplane 94 and at an angle of 40 with respect to first direction 116. Whenmicromirror 56 is in its second position, as indicated by dotted line92, the incoming light impinging from the second direction is reflectedat an angle of 90 with respect to plane 94 and thus passes throughprojecting system 120 to screen 122. This is indicated by an arrow 138,which is coaxial with arrow 118. However, when micromirror 56 is in itsfirst position, indicated by line 90, light impinging from the seconddirection (i.e., the direction marked by arrow 136) is deflected in adirection shown by arrow 140 to an absorber 142. Absorber 142 is locatedat an angle of 50 with respect to plane 94.

To recapitulate, from FIG. 4 it will be apparent that red or green lightimpinging from a first direction marked by arrow 116 is reflected in thedirection of arrow 118 when micromirror 56 is in its first position andthat blue tight impinging from a second direction marked by arrow 136 isreflected in the direction of arrow 138 when micromirror is in itssecond position. Arrows 136 and 118 are coaxial, and both representlight which is directed toward projection system 120.

Returning to FIG. 1, display apparatus 20 also includes a bias and resetunit 144 which operates under the control of control unit 38 to supplythe bias voltage and resonant reset pulses as previously discussed. Abuffer 146 detects when the last row electrode 54 has been strobed andprovides a signal to an exposing or lighting unit 148. The red flashtube 96, green flash tube 102, and blue flash tube 128 are part of thelighting unit 148 and serve as an illumination unit 150. Lighting unit148 also includes a delay 152, a trigger unit 154, a selector 156, anaddress counter 158, a control ROM 160, a flash timer 162, and a lampdriver unit 164. Lamp driver unit 164 includes a high voltage source 168which charges a capacitor 170 through a resistor 172 in order to supplyenergy to illumination unit 150 when one of the red, green, or blueflash tubes 96, 102, or 128 is fired, and a quenching circuit 174 whichterminates the flash after a duration established by the flash timer162.

The operation of display apparatus 20 is shown in the flow chart of FIG.5. After one frame has been displayed, a new frame is stored in step 176by transferring the red component of the new frame from memory 32 tomemory 40, by transferring the green component of the new frame frommemory 34 to memory 42, and by transferring the blue component of thenew frame from memory 36 to memory 44. Memory 40, for example, storesvideo words corresponding in number and arrangement to the number andarrangement of micromirrors 56 in the DMD 46. Each of these video wordshas a least significant bit, a most significant bit, and at least oneintermediate bit having a rank between that of the least significant bitand the most significant bit. Memories 42 and 44 are similar, exceptthat they store video words for the green and blue components of theimage.

Memory 40 is selected in step 178, and address counter 158 is cleared to0 in step 179. A bit-rank counter in control unit 38 is set to 1,indicating the least significant bit, in step 180. Address counter 158is cleared to 0 in step 182.

The bit rank designated by the bit rank counter is read into DMD 46during 5 step 184. When the bit rank counter is set to the leastsignificant bit, this means that the least significant bit for all thevideo words which are stored in memory 40 (and which correspond torespective micromirrors 56) are transferred to memory cell andmicromirror array 58. This is accomplished by transferring the leastsignificant bits for a first raster line to serial/parallel converterand register 48 and strobing a first row electrode 54, transferring theleast significant bits for a second raster line to converter andregister 48 and strobing a second row electrode 54, and so forth untilthe least significant bits for the last raster line for the redcomponent are transferred to converter and register 48 and the last rowelectrode 54 is strobed. Thereafter the program waits at step 186 for aperiod which permits lighting unit 148 to fire red flash tube 96 at alow level. This will be discussed in more detail later. Resonant resetpulses are supplied from unit 144 during step 188 in order to dislodgeany micromirrors 56 that may have become stuck. Then a check is made atstep 190 to determine whether the content of the bit rank counter incontrol unit 38 is equal to the most significant bit. If not, the bitrank counter is incremented at step 192 and the program returns to step184. In the second repetition, the bits of the red component having arank just above the least significant bits are transferred to array 58in step 184, and the program waits at step 186 while lighting unit 148administers a red flash having twice the light that was emitted duringthe flash for the least significant bits. Steps 184–190 are performeduntil the most significant bits of the red component have been displayedon array 58 and exposed to a red flash at a level commensurate with themost significant bit.

After the most significant bits of the red component have been displayed(“YES” at step 190), a check is made at step 192 to determine if memory42, which stores the green component, has already been selected. If not,it is selected at step 194 and the program returns to step 180. The bitsof the green component are displayed rank-by-rank, from the leastsignificant bit to the most significant bit, while DMD 46 receivesflashes of green light of increasing binary levels from lighting unit148. If memory 42 has already been selected (“YES” at step 192), a checkis made at step 196 to see whether memory 44 has been selected. If not,memory 44 is selected at step 198 and the blue component of the frame isdisplayed on DMD 46 rank-by-rank white lighting unit 148 suppliesflashes of blue light at levels commensurate with the bit ranks whichare being displayed. After the blue component has been displayed (“YES”at step 196), the program returns to step 176 and the red, green, andblue components of the next frame are displayed in the same way.

What happens during the wait at step 186 will now be described in moredetail.

When the last row electrode 54 is strobed during storage of the leastsignificant bits of the last raster line of the red component, buffer146 supplies a pulse to lighting unit 148. This pulse is counted byaddress counter 158, which was cleared at step 179 in FIG. 5 and thusprovides an output of 00 . . . 1 as an address signal to control ROM160. ROM 160 stores lighting control words which determine which of thered, green, and blue flash tubes is selected and the level at which theselected flash tube is flashed. Each lighting control word includes a3-bit color selection portion which is supplied via a bus 200 toselector 156 and a multi-bit light level portion which is supplied via abus 202 to flash timer 162. The light level portion determines how longa flash is to last. The flash from a least significant bit is brief. Forthe next-least significant bit, the duration established by the lightlevel portion provides a flash with twice the total amount of light thatwas emitted during the flash for the least significant bit. For the nextleast significant bit, the duration provides a flash with four times thetotal amount of light as the flash for the least significant bit. Thedurations of the flashes for higher-order bits are set in a similar way,with the amount of tight liberated during a flash being commensuratewith the rank of a bit. The color selection portion of the lightingcontrol word is 001 if red flash tube 96 is to be selected, 010 if greenflash tube 102 is to be selected, and 100 if blue flash tube 128 is tobe selected.

The pulse from buffer 146 is delayed by the delay in unit 152 and thensupplied to trigger unit 154, which generates a trigger pulse that isdelivered to selector 156. Since the color selection portion of thelighting control word stored at location 00 . . . 1 in ROM 160 is 001,selector 156 forwards the trigger pulse to the trigger terminal 204 ofred flash tube 96. This initiates a flash, using the energy stored oncapacitor 170. The trigger pulse is also supplied via line 206 to flashtimer 162, which begins timing the flash. Although not shown, flashtimer 162 may include an oscillator, a counter which begins countingpulses from the oscillator when it receives the trigger signal on line206, a register which holds the light level portion of the lightingcontrol word received from ROM 160, and a comparator which signalsquenching circuit 174 when the content of the counter reaches the valuestored in the register.

After the flash for the least significant bit of the red component,further flashes for the red component are orchestrated under the controlof subsequent lighting control words read out of ROM 160. The colorselection portion of the lighting control words for all of the redflashes is 001 but the light level portions command different amounts oflight that increase in a binary manner as indicated above. After theflashes of the red component, the flashes for the green and bluecomponents are generated in a similar manner.

With reference next to FIGS. 1 and 4, it will be recalled that the redand green flashes impinge on DMD 46 from a first direction 116. When aparticular bit of the red or green component is logical 1, themicromirror 56 which is influenced by that bit is moved to the positionshown by line 90 and reflects the impinging light in the direction ofarrow 118 to projecting system 120. However the blue flashes impinge onDMD 46 from a second direction marked by arrow 136, and when themicromirror 56 is in the position marked by solid line 90 these flashesare diverted to absorber 142. This is the reason why an inverter 208 isshown in FIG. 1 to invert the blue component stored in memory 44 beforethe blue component is transferred to converter and register 48. When aparticular bit of the actual blue component is logical 1, meaning that aspot of blue light should be displayed on screen 122, inverter 208inverts the bit to logical 0 in order for the corresponding flash ofblue light to be reflected in the proper direction for display. That is,the position of a micromirror 56 indicated by solid line 90 is a displayposition for red and green dots but not for blue dots, and the positionshown by dotted line 92 is a display position for blue dots but not forred or green dots.

It will be apparent to those skilled in the art that some of thefunctions performed by lighting unit 148 can be transferred to controlunit 38. For example, lighting control words can be dealt out by amicroprocessor in control unit 38, making counter 158 and ROM 160unnecessary. It will also be apparent that unit 144 may apply andrelieve the bias voltage in a manner which electromechanically latchesthe micromirrors 56 until they can all be updated at once at theconclusion of step 184. However, simultaneous updating of themicromirrors 56 is not necessary since it is only their positions at thetime of a flash that counts. That is, it is sufficient if all of themicromirrors are in their updated positions at the end of step 184.

The Second Embodiment

FIG. 6 illustrates a portion of the lighting unit in accordance with asecond embodiment of the display apparatus of the present invention.This embodiment has a modified illumination unit 150′ and not doesrequire flash timer 162. Instead, the light level portions of thelighting control words are supplied from ROM 160 (see FIG. 1) to a D/Aconverter 210.

Illumination unit 150′ includes lamps such as a red flash tube 96, agreen flash tube 102, and a blue flash tube 128 which receive triggersignals from selector 156 (see FIG. 1). The color selection portions ofthe lighting control words are supplied to selector 156 as in the firstembodiment, and are also supplied via a bus 200 to switches 212, 214,and 216. A light sensor 218 and an amplifier (not numbered) areconnected to the input side of switch 212; a light sensor 220 and anamplifier (not numbered) are connected to the input side of switch 214;and a light sensor 222 and an amplifier (not numbered) are connected tothe input side of switch 216. The color selection portion of thelighting control word causes one of the switches 212, 214, and 216 toclose before the corresponding flash tube is flashed, thereby selectingwhich sensor signals are supplied to an integrator 224 during a flash. Acomparator 226 emits a signal on line 228 to quench circuit 174 (seeFIG. 1) when the total amount of light sensed during a flash is equal tothat designated by the light level portion of the lighting control word.

In the first embodiment, the flash duration was controlled in order toset the total amount of light emitted during each flash. However, in thefirst embodiment the relation between flash duration and the totalamount of light must be determined experimentally for a flash tube of aparticular type before appropriate light level portions can be stored inROM 160. The illumination unit 150′ of the present embodiment alleviatesthis problem by measuring the flashes as they occur and comparing theintegrated flashes with the desired light level.

The Third Embodiment

FIG. 7 illustrates a portion of an exposing or lighting unit inaccordance with a third embodiment of the present invention. Thislighting unit includes a modified lamp driver unit 164′ which suppliespower to the illumination unit 150 (see FIG. 1) via a line 230. Flashtimer 162 is unnecessary in this embodiment. Lamp driver unit 164′includes a latch 232 which receives the light level portion of thelighting control words from ROM 160 (see FIG. 1) via bus 202. The lightlevel portion itself in this embodiment has a three-bitcapacitance-selection portion which determines which of switches 234,236, and 238 will be closed and a multi-bit charging-voltage portionwhich is supplied via a bus 240 to a D/A converter 242. D/A converter242 supplies an analog signal to a control input port of a high voltagesource 244, which charges capacitors 246, 248, and/or 250, dependingupon which of switches 234–238 is closed, to the voltage set by thecharging-voltage portion of the lighting control word.

Suppose that the capacitance of capacitor 250 is C units, that thecapacitance of capacitor 248 is also C units, and that the capacitanceof capacitor 246 is 2C units. This means that the total capacitance is Cwhen switch 238 is closed, 2C when both the switches 236 and 238 areclosed, and 4C when all three switches are closed.

The energy E stored on a capacitor having capacitance C is given by thefollowing equation:E=½CV ²Accordingly, for a given charging voltage V, one unit of energy can bestored by closing switch 238, two units of energy can be stored by alsoclosing switch 236, and four units of energy can be stored by alsoclosing switch 234. To store eight units of energy, only capacitor 250would be charged, but it would be charged to a voltage that is largerthan the original voltage V by a factor of 2√2. Sixteen units of energycan be stored by charging both capacitors 248 and 250 to the highervoltage, 2√2V. Thirty-two units of energy can be stored by charging allthree capacitors to the higher voltage. This is summarized in thefollowing Table:

TABLE Energy Capacitance Voltage 1  C V 2 2C V 4 4C V 8  C 2Vsqrt2 16 2C2Vsqrt2 32 4C 2Vsqrt2

It will be apparent from the foregoing that the energy emitted during aflash 20 can be controlled by designating the total capacitance usingthe capacitance selection portion of the lighting control word and thevoltage using the charging voltage portion of the lighting control word.Even the simple example illustrated in the Table above shows that sixbinary levels are readily available, and more can easily be added byincreasing the number of capacitors or the number of voltage levels towhich they are charged.

From the discussion of the first embodiment it will be recalled thataddress counter 158 (see FIG. 1) supplies a new address to control ROM160 each time the last row electrode 54 is strobed. In the thirdembodiment, however, it is desirable to let the selected capacitor orcapacitors charge while new values are being read into array 58, and notto change the selection of capacitors or the charging voltage untilafter the flash has been delivered. For this reason, in the thirdembodiment control unit 38 emits a latch signal on line 252 when itbegins reading a new bit rank into DMD 46 (step 184 in FIG. 5), at whichpoint the lighting control word supplied by ROM 160 is latched. A newlighting control word is read out of ROM 160 when the last row electrode54 is strobed, and the color selection portion of this new lightingcontrol word determines which of the flash tubes 96, 102, and 128 isflashed, but the light level portion designated by the old lightingcontrol word remains effective until after the flash is generated.

The Fourth Embodiment

A fourth embodiment of the display apparatus in accordance with thepresent invention will now be described with reference to FIGS. 8, 9,and 10A–10C. In this embodiment, an input unit 254 supplies digitalizedred, green, and blue video words to a digital micromirror device or DMD256. DMD 256 includes a memory cell and micromirror array 258. Thedisplay apparatus of this embodiment also includes a reset circuit 260and an exposing or lighting unit 262.

The lighting unit 262 includes an illumination unit 264 having a redfluorescent lamp 266, a green fluorescent lamp 268, and a bluefluorescent lamp 270. Switches 272, 274, and 276 in illumination unit264 selectively connect the lamps to a lamp driver unit 278. A colorregister 280 receives a three-bit color selection signal from input unit254 to control which of the switches is closed. Switch 272 is closed toconnect red lamp 266 to lamp driver unit 278 when the color selectionsignal is 100; switch 274 is closed to connect green lamp 268 to lampdriver unit 278 when the color selection signal is 010; and switch 276is closed to connect blue fluorescent lamp 270 to lamp driver unit 278when the color selection signal received from input unit 254 is 001.

Lighting unit 262 also includes an intensity register 282, whichreceives a multi-bit light intensity signal from input unit 254. Thelight intensity signal signifies how intensely the selected lamp is tobe driven. The light intensity signal stored in register 282 isconverted to analog by a D/A converter 284 and then supplied to acontrol input port of an intensity controller 286. Controller 286steadily drives the selected lamp at the desired intensity by varyingits duty cycle. In this application, “steady” emission of light by aselected lamp during an interval means light that is emitted throughoutthe interval, even if the light may be periodically interrupted duringthe interval due to duty cycle control of the intensity of theillumination.

In the embodiments previously described, a negative bias voltage wassupplied to the landing electrodes and the micromirrors toelectromechanically latch the micromirrors into place while the memorycells were being updated, the bias voltage being briefly releasedthereafter to permit the micromirrors to move to their new statessimultaneously. In contrast, in the present embodiment, the landingelectrodes and the micromirrors are kept at ground potential (or at anegative potential whose absolute value is insufficient to causeelectromechanical latching). Each row of micromirrors is therefore freeto respond while the memory cells beneath them are updated. If light isshining on the micromirrors at a time when all of them are positioned tobe OFF and if the micromirrors are then moved to their ON positions on arow-by-row basis from the first row to the last row, it is apparent thatthe first row of micromirrors receives more light than the last row. Butif the micromirrors are then turned OFF from the first row to the lastrow, at the same rate they were previously turned ON, they will remainin the ON state for the same amount of time and consequently receive thesame amount of light if they are steadily illuminated. When the turn-onand turn-off periods are considered together, each row receives half ofthe total amount of light emitted during the two periods.

The operation of this embodiment will now be described. It will beassumed that the red, green, and blue video words which DMD 256 receivesfrom input unit 254 each have six bits. The discussion will start withthe display of the least significant bit of the red component of theframe.

Input 254 supplies color register 280 with a color control signal havinga value of 100, thus causing switch 272 to close and connecting red lamp266 to lamp driver unit 278. Input unit 254 also supplies register 282with a light intensity signal which causes controller 286 to vary theduty cycle of lamp 266 so as to drive it at a low level L (see FIG.10C). Then input unit 254 supplies the least significant bits of the redcomponent row-by-row to DMD 256 during a turn-on period marked byreference 288 number in FIG. 10B. Turn-on periods in FIG. 10B, it isnoted, are designated by upward arrows. Thereafter there is a briefresetting period 290 (the duration of which is much exaggerated in FIG.10B) followed by a turn-off period 292 during which the micromirrors areturned off row-by-row at the same rate they were turned on previously(turn-off periods are identified by downward pointing arrows in FIG.10B, and have the same duration as the turn-on periods). This completesthe display of the least significant bit, as indicated in FIG. 10A.There follows a resetting period 294. As is the case with the otherresetting periods shown in FIG. 10B, the duration of period 294 isexaggerated considerably in the drawing.

The display of the second bit begins with a turn-on period 296 followedby a maintain period 298 of the same duration. It is noted that maintainperiods are designated in FIG. 10B by horizontal arrows. After aresetting period (not numbered), the display of the second bit isconcluded by a turn-off period 300. Each row of micromirrors receivesthe same amount of light during the maintain period 298 as it doesduring the turn-on and turn-off periods 296 and 300 together. Thus, thetotal amount of light received during display of the second bit by eachrow of micromirrors is twice that received during display of the leastsignificant bit. This can be designated as 2L, in contrast to an amount1L of light received during display of the least significant bit.

The maintain period 302 during display of the third bit is three timesas long as the turn-on period 304 or the turn-off period 306.Consequently, the amount of light received by each row of micromirrorsduring display of the third bit is 4L.

At this point, input unit 254 supplies register 282 with an intensitysignal which causes the intensity of the red light to increaseeight-fold, to a new level H. The display of the fourth bit isaccomplished with a turn-on period and a turn-off period but no maintainperiod. In this respect, the display of the fourth bit is the same asthe display of the least significant bit, except the illumination is nowat a high level H=8L. Thus, twice as much light is received by each rowof micromirrors during display of the fourth bit as was received duringdisplay of the third bit.

Display of the fifth bit and the sixth or most significant bit is thesame as the display of the second and third bits, except at the higherintensity level H. After the most significant bit of the red componenthas been displayed, the green and blue components are displayed in asimilar manner.

Although a biasing potential is not used to latch the micromirrors inthis embodiment, it is still necessary to be concerned about sticking ofthe micromirrors due, perhaps, to cold welding. A torroidal magnet 304is disposed around DMD 256 to create a magnetic field in the region ofthe micromirrors. This field has a vertical component (with respect toFIG. 8) which passes through array 258. As is shown schematically inFIG. 9, each row of micromirrors is connected to the reset circuit 260.Input unit 254 supplies a signal to reset circuit 260 during theresetting periods (e.g., 290, 294, and so forth), whereupon resetcircuit 260 emits a pulse of electrical current which flows through eachrow of micromirrors. The interaction of the current and the magneticfield established by magnet 304 generates a Lorentz force which jostleseach micromirror and thereby dislodges any that have become stuck. Ifdesired, a series of current pulses can be generated by reset circuit260 during each reset period, the series of pulses having a frequencymatching the resonance frequency of the micromirrors.

As will be noted from FIG. 9, in this embodiment the micromirrors ofalternate rows are offset with respect to the column direction. Tocompensate for this, it is desirable for the color signals for alternateraster lines to be shifted in phase by half of the pitch of themicromirrors before they are supplied to input unit 254. However if allof the input signals have the same phase, the values for adjacent pixelsof alternate raster lines can be averaged in unit 254 before beingsupplied to DMD 256.

The Fifth Embodiment

The fifth embodiment employs a lighting unit 262 (see FIG. 8), as in thefourth embodiment. Unlike the fourth embodiment, though, the fifthembodiment dispenses with the reset circuit 260 and torroidal magnet 304that were employed in the fourth embodiment. Instead, the fifthembodiment uses conventional means (such as resonant repulses) todislodge any micromirrors that have become stuck, and conventionalbiasing to electromechanically latch the micromirrors as the memorycells beneath them are being updated, as in the first embodiment (e.g.,bias and reset unit 144 in FIG. 1).

Since electromechanical latching of the micromirrors is employed in thefifth embodiment, all of the micromirrors are updated simultaneouslywhen a new bit rank of the video words is displayed. This, of course,makes it unnecessary to turn the micromirrors off, as in the fourthembodiment, before proceeding to a new bit rank. The turn-off periodsshown in FIG. 10B (marked by downward arrows) can therefore be omitted.The result is that the least significant bits are displayed as the DMDis illuminated at the low level L for a predetermined period of time.The micromirrors are updated in accordance with the second leastsignificant bits, and the DMD is illuminated at the level L for twicethe predetermined period. The micromirrors are updated in accordancewith the third least significant bits, and the DMD is illuminated at thelevel L for four times the predetermined period. The micromirrors areupdated in accordance with the fourth least significant bits, and theDMD is illuminated at the high level H for the predetermined period oftime. The micromirrors are updated in accordance with the fifth leastsignificant bits, and the DMD is illuminated at the H level for twicethe predetermined period. The micromirrors are updated in accordancewith the most significant bits, and the DMD is illuminated at the Hlevel for four times the predetermined period. That is, FIGS. 10A and10C remain unchanged in the fifth embodiment.

The Sixth Embodiment

In an embodiment shown in FIG. 11, micromirrors 306 in a first row aresupported by posts 308 in the first row and micromirrors 310 in a secondrow are supported by posts 312 in the second row. However micromirrors314 in an intermediate row do not have their own posts, but are insteadsupported by the posts 308 and 312 of the first and second rows. As aresult of this arrangement, the micromirrors 306 and 310 pivot aboutaxes 316 while the micromirrors 314 of the intermediate row pivot aboutorthogonal axes 318.

Fluorescent lamps are used in this embodiment and they steadily emitlight when selected, as in the fourth embodiment. Their geometry isdifferent from that shown in FIG. 4 in the following respects: The redand blue fluorescent lamps 266 and 270 are oriented the same way as thecorresponding red and blue lamps 96 and 128 in FIG. 4. Red lighttherefore impinges on the DMD from a first direction 116 and blue lightimpinges on it from a second direction 136 (the video words for the bluecomponent of the image are inverted, of course, as in the firstembodiment). The first and second directions 116 and 136 lie in a commonplane which is perpendicular to the pivot axes 318 shown in FIG. 11.Thus, when the micromirrors 314 are pivoted to the right (with respectto FIG. 11) they can display red dots on screen 122, and when they arepivoted to the left they can display blue dots.

The green fluorescent lamp is located in a plane which is perpendicularto the pivot axes 316 and thus to the common plane of the first andsecond directions 116 and 136. That is, the position of green lamp 102in this embodiment would not be the same as shown in FIG. 4, and halfmirror 110 would be unnecessary. When micromirrors 306 and 310 arepivoted upward (with respect to FIG. 11) they display green dots andwhen they are pivoted downward they are OFF.

From this arrangement, every other row of micromirrors provides a rasterline which displays both the red and blue components of the image andthe remaining micromirrors provide raster lines which display the greencomponent. Consequently two rows of micromirrors are needed to displayall three color components, and thus the resolution attainable by thisembodiment is not as large as that afforded by previous embodiments.However, this embodiment has two significant advantages. One is that thelight loss associated with half mirror 110 in FIG. 4 is avoided. Theother is that the green component can be displayed by the micromirrors314 while either the red or the blue component is displayed by themicromirrors 306 and 310. Because of this, the lamps for the red andblue components can be active for approximately half of the frame periodrather than a third, and the lamp for the green component can be activefor approximately the entire frame period. Consequently the power of thelamps can be reduced.

A modification of the resetting arrangement employed in the fourthembodiment can also be used with this embodiment. A magnet 304 would beused, as would a reset circuit which emits current pulses to generate aLorentz force. The current pulses would be applied horizontally throughrows which include micromirrors 306 and 310 and then vertically throughcolumns which include micromirrors 314.

The two-axis arrangement shown in FIG. 11 can also be used for opticalswitching of two optical signals that can be distinguished from oneanother, as by using orthogonal polarizations. For example, themicromirrors 306 and 310 could be used to switch light having a firstpolarization up or down, and the micromirrors 314 could be used toswitch light having a second polarization left or right. A single DMDcould be divided into clusters of micromirrors, with light that has beenswitched by one cluster being input to another cluster for the purposeof further switching.

The Seventh Embodiment

A display apparatus 320 in accordance with the seventh embodiment isshown in FIG. 12, and includes an input unit 322 having an inputterminal 324 for receiving video words for the red component of animage, an input terminal 326 for receiving video words for the greencomponent, an input terminal 328 for receiving a video words for theblue component, and an input terminal 330 for receiving synchronizationsignals. Each video word specifies one of a plurality of binary levelsfor the red, green, or blue intensity of a corresponding pixel that isto be displayed. The video words for the red, green, and blue componentsare stored in respective frame memories 332, 334, and 336 under thecontrol of a control unit 338, which includes a microprocessor. When afull frame is stored, control unit 338 transfers the contents ofmemories 332–336 to further frame memories 340, 342, and 344, and thenbegins storing the next frame in memories 332–336. Control unit 338 alsoreads out the contents of memories 340–344 to a display unit having anaddressable spatial light modulator with an array of bi-stable (that is,either on or off) pixels. In this embodiment, the display unit is adigital micromirror device (DMD) 346.

The addressing means for DMD 346 includes a serial/parallel converterand register 348 which receives a series of bits as input data andadjusts the voltages on column conductors 350 in accordance with theinput data. The addressing means also includes a gate decoder 352 whichstrobes row electrodes 354 in sequence. Each time a row electrode isstrobed, the data on the column electrodes 350 are stored in a row ofstatic memory cells corresponding to the row electrode. A micromirror isdisposed above each memory cell and serves as a pixel that is controlledby the memory cell. The memory cells and micromirrors together form anarray which is designated by reference number 356 in FIG. 12.

A lighting unit 358 exposes the micromirrors to red, green, and bluelight having different intensity levels as the micromirrors are turnedon and off to build up a frame image. A “frame image” is intended torefer to what is to be displayed by the pixels of all of the rows ofmicromirrors that are to participate in forming an image during any onescanning cycle of array (that is, a frame image consists of the pixelsof all of the rows in array 356 if progressive scanning is used, andalternating rows if interlaced scanning is used). In what follows, itwill be assumed that progressive scanning is employed, so that a frameimage represents a complete snapshot of what is being displayed. Thelighting unit 358 includes a monitor unit 360, an illumination unit 362,an intensity register 364, and a lamp driver unit 366.

The illumination unit 362 includes a color wheel 368, which is rotatedby a motor 370 that is controlled by a motor control unit 372. A lampunit 374 is disposed in a housing 376. The lamp unit 374 has alow-intensity lamp 378 and a high-intensity lamp 380. The intensity oflamp 380 is seven times greater than that of lamp 378. That is, if lamp378 has an intensity of one in arbitrary units, lamp 380 has anintensity of seven, and both lamps together have an intensity of eight.An optical system 382, which is illustrated only schematically,collimates light from the lamp unit 374.

Referring next to FIGS. 12 and 13 together, the color wheel 368 includesa frame 384 that supports a red filter 118R, a green filter 118G, and ablue filter 118B. The width of the arms of frame 384 will be generallyignored in what follows and, for convenience, it will be said that eachof the colored filters provides a colored sector that extends(approximately) 120° The red sector begins at 0°; the green sectorbegins at 120°; and the blue sector begins at 240°. Motor control unit372 generates angle information that is supplied to control unit 338 viaa line 388. The angular information may be a train of pulses that aregenerated by a sensor (not illustrated) in the control unit 372, thesensor being linked to the motor's shaft. Once every revolution of colorwheel 368, at the 0° mark, the motor control unit also generates astart-of-revolution signal (such as a long pulse) that is supplied tocontrol unit e38 as part of the angular information. By counting pulsesafter the start-of-revolution signal, the control unit 338 is informedabout which color sector is currently active, and how far that colorsector has progressed.

The intensity register 364 in FIG. 12 receives a one-bit light intensitycommand signal from control unit e38 via a line 390, and the lamp driverunit 366 drives lamp unit 374 accordingly. The intensity command signalspecifies either a low-light level (when the light-intensity commandsignal is 0), in which case only the low-intensity lamp 378 is driven,or a high-light level (when the light-intensity command signal is 1), inwhich case both the low-intensity lamp 378 and the high-intensity lamp380 are driven to produce a total intensity of eight. The low-intensitylamp 378 is thus always on, while the high-intensity lamp 380 turns onand off.

The monitor unit 360 includes a light sensor 392 which senses theintensity of the light passing through color wheel 368, and generates acorresponding signal that is supplied to an amplifier 394 and thence toan analog-to-digital converter 396. The digital value of the sensedlight intensity is then supplied to an integrator 398, which can bereset to zero by control unit 338 via a line 400. A light-level register402 receives a multi-bit light-level integration value via a line 304from control unit 338, and supplies it to a comparator 406, which sendsa level-reached signal to control unit 338 via line 408 when the outputof integrator 400 reaches the light-level integration value held inregister 402. At this point, it is appropriate to note that the lightintensity command that is received by register 364 is not the same asthe light-level integration value that is received by register 402. Thelight intensity command indicates the instantaneous intensity that isdesired—that is, whether only the low-intensity lamp 378 should bedriven or whether the high-intensity lamp 380 should also be driven. Thelight-level integration value, in contrast, indicates the total amountor quantity of light that is desired, that is, the intensity times itsduration.

Not yet mentioned in FIG. 12 is a bias and reset unit 410, whichoperates under the control of control unit 338 to supply the biasvoltage and resonant reset pulses, as previously discussed. For purposesof the present invention, however, it is only necessary to consider thebias voltage, which is applied to array 356 to latch the micromirrorsinto their current positions as new data is being read into DMD 346, andis then temporarily relieved to permit the micromirrors to be moved intotheir new positions, whereupon the bias voltage is reapplied to latchthe micromirrors at their new positions.

The operation of this embodiment will now be described with reference toFIG. 12 and the flowchart shown in FIGS. 14A and 14B. After one framehas been displayed, a new frame is stored in step 412 by transferringthe red component of the new frame from memory 332 to memory 340, bytransferring the green component of the new frame from memory 334 tomemory 342, and by transferring the blue component of the new frame frommemory 336 to memory 344. Memory 340, for example, stores video wordscorresponding in number and arrangement to the number and arrangement ofmicromirrors in the DMD 346. In this example, each of the video wordshas seven bits. Memories 342 and 344 are similar, except that they storevideo words for the green and blue components of the image.

Memory 340 for the red component is selected in step 414. A bit-rankcounter (not illustrated) in control unit 338 is set to zero, meaningthe least significant bits of the red component, in step 416. The leastsignificant bits for the video words of the red component are then readinto DMD 346 during step 422.

In step 420, a check is made to determine whether the color wheel 368 ispositioned at the beginning of its red sector (that is, 0°). When thecolor wheel reaches the beginning of the color sector, control unit 338loads a light-level integration value for the bit rank designated by thebit rank counter into the light-level register 402 (step 154). Since thebit rank counter was set at zero in step 416, the integration valueloaded into register 402 during the first repetition designates thelight level for exposing the pixels during display of the leastsignificant bits. For convenience, this light level will be said to be“1” in arbitrary units. Then control unit 338 signals bias and resetunit 410 to latch the data read at step 418 into the DMD 346 (step 156).In the first repetition of the program's steps, the micromirrors thusmove to their positions for displaying the least significant bits of thered component of the image. Control unit 338 resets integrator 398 tozero in step 426. Consequently, the integrator 398 starts integratingthe signal from light sensor 392. Control unit 338 increments the bitrank counter in step 428, and then reads the bit rank designated by thebit rank counter (LSB+1 during the first repetition) into the DMD 346during step 430.

At the conclusion of step 430, new data has been read into the memorycells of DMD 336, but the micromirrors are still latched at their oldpositions, and integrator 398 is still integrating toward thelight-level integration value for the previous bit rank. When thisintegration value is finally reached (step 432), a check is made to seewhether the bit rank counter has been incremented to a value greaterthan 2 (step 434). If not, the program returns to step 422, and register402 is loaded with the light-level integration value for the bit rankdesignated by the bit rank counter. The micromirrors are then latched atstep 424 in accordance with the bit rank read into the bit rank counterin step 430, and steps 426–432 ensue.

In FIG. 15, the least significant bits of the video words of the redcomponent are displayed during the period from T₀ to T₁. From theexecution of step 412 until the return to step 412, the light intensityis 1 since low-intensity lamp 378 is always on. The next bits (LSB+1)are displayed during the period T₁ to T₂. They are displayed twice aslong as the least significant bits because the light-level integrationvalue for the second bits is twice as large as that for the leastsignificant bits. The light-level integration value for the next bits,which are displayed from T₂ to T₃, is four times as large as that forthe least significant bits, and therefore the pixels are exposed tolight at intensity one during display of the third bits (LSB+2) for aperiod that is four times as long as the least significant bits.

Returning now to step 432 in FIG. 14A, when the bit rank counter hasbeen incremented to a value greater than two, a check is made at step436 to determine whether the high-intensity lamp 380 has already beenturned on. If not, it is turned on in step 438. FIG. 15 shows atransition region 440 when this occurs. The intention in FIG. 15 is notto show the actual turn-on behavior of lamp 380, which would depend uponthe exact type of lamp and its age and upon the particular nature ofdriver unit 336, but rather to indicate schematically a build-up periodbefore lamp 380 reaches its fill intensity. That is, the presentinvention does not demand a high-intensity lamp 380 that is capable ofsnapping full-on instantaneously. Rather, erratic or unruly behavior canbe tolerated in transition region 440 (and, indeed, outside of thetransition region) because the actual illumination is sensed andintegrated.

A check is made at step 442 to determine whether the bit rank counterhas been incremented to 6 (the most significant bit, since the videowords have seven bits in this example). If not, the program returns tostep 422, and LSB+3, LSB+4, and LSB+5 are displayed, as shown in FIG. 6.If the bit rank counter does indicate the most significant bit, however,the light-level integration value for the most significant bit is loadedinto register 402 at step 444. The micromirrors are then latched intotheir positions for displaying the most significant bits of the redcomponent in step 446, and integrator 398 is reset to zero in step 448.While integrator 398 is integrating toward the light-level integrationvalue for the most significant bits, zeros are read into the DMD 346(step 450). A zero indicates the off position for a micromirror. Whenthe integration value for the most significant bits is reached (step452), the micromirrors are latched at their off positions (step 454).The high-intensity lamp 380 is then turned off in step 456, leaving onlythe low-intensity lamp 378 illuminated. FIG. 15 shows a transitionregion 458 back to a light-intensity level of one. The changing lightintensity in transition region 458 does not matter, since zeros aredisplayed during the period from T₇ to T₈.

The period from T₇ to T₈ is very important since it acts as a sponge toabsorb variations in the turn-on behavior of high-intensity lamp 380(transition region 440) and variations in the level attained by lamp 380when it is fully on. As lamp 380 ages, for example, its intensity mightchange from seven times that of the low-intensity lamp 376 to six timesthe intensity of lamp 378, and this would alter the locations of thetimes T₄–T₇ in FIG. 15. The time T₈ needs to be set far enough down thetime axis that T₇ does not overtake T₈ while the lamps are operating inaccordance with their design specifications. The time between T₇ and T₈when DMD 346 displays all zeros and is effectively off can be termed a“buffer period” which, in conjunction with the sensing and integrationof the light impinging on DMD 346, absorbs variations in the lightproduced by lamp unit 374 and thus tolerates less than perfect behaviorby lamp unit 374.

The display of the red component of the image is complete when step 454is executed. The angle signal emitted to control unit 338 by motorcontrol unit 372 at this point is less than 120°. The color wheel 368continues turning during the buffer period between T₇ and T₈. At step460, a check is made to determine whether memory 432 for the greencomponent of the image has already been selected. If not, it is selectedat step 462, and the program returns to step 416 to display these sevenbits of the video words for the green component of the image. In thefirst repetition of the program's steps during the green display, thefilter is deemed to be OK (step 420) at the beginning of green sector386G (that is, when the color wheel reaches 120°). After the greencomponent of the image has been displayed, a check is made at step 464to determine whether the memory 344 for the blue component has alreadybeen selected. If not, it is selected in step 466, and the bluecomponent is subsequently displayed (steps 416–454). If the memory 344has indeed already been selected, the program returns to step 412 todisplay the next frame.

Although color wheel 368 is used in FIG. 12 to color the light from lampunit 374 before the light impinges on DMD 346, the color wheel 368 couldbe used instead to color the light after reflection by the micromirrors.The sensor 392, however, should measure the light before impingement onthe DMD 346 since it would otherwise be necessary to correct the sensedamount of light in accordance with the on/off states of themicromirrors.

The Eighth Embodiment

The eighth embodiment is also based on the structure shown in FIG. 12.This structure is controlled in a different manner, however, to reducethe frequency at which the high-intensity lamp 380 is turned on and off.

In FIG. 16, the red, green, and blue video words for the next frame arestored at step 468. Then, in step 470, the least significant bits andthe next-to-least significant bits (LSB+1) are displayed for all threecolors during a first revolution of the color wheel 368 (the details ofstep 470 will be described later with reference to FIG. 18A). This isshown schematically in FIG. 17A, which illustrates the three coloredfilters 386R, 386G, and 386B of the color wheel 368, and additionallyindicates the angular segments through which the filters rotate duringthe display of the least significant bits and LSB+1. The cross-hatchedregions in FIG. 17A indicate buffer periods during which the DMD 346displays all zeros (that is, all of the micromirrors are in their offpositions), and thus all of the pixels are dark. Only the low-intensitylamp 378 is on during the display of the LSB and LSB+1.

In step 472, the bits LSB+2 for all three colors are displayed during asecond revolution of the color wheel 368, again with only thelow-intensity lamp 378 being illuminated. This is shown in FIG. 17B. Asbefore, the cross-hatched buffer periods in FIG. 17B indicate that theDMD 346 displays all zeros.

In step 474, the high-intensity lamp 380 is turned on, so that it shinesalong with the low-intensity lamp 378. As the intensity of lamp 380rises, in the transition region 440 shown in FIG. 15, the DMD 346displays all zeros (step 476) during a third revolution of color wheel368. This is shown in FIG. 17C. Since the brightness of lamp 380 isselected to be seven times as great as that of lamp 378 when lamp 380 isfully on, the total intensity at the end of the third revolution iseight times as high as that during the first revolution (FIG. 17A). Thedisplay unit is now ready to display the LSB+3 and LSB+4 bits for allthree colors during the fourth revolution (step 478). This isillustrated in FIG. 17D. The similarity between FIGS. 17D and 17A shouldbe noted, with the difference being that the light is eight times asbright in FIG. 17D.

Next, in step 480, the bits LSB+5 are displayed for all three colors.This is shown in FIG. 17E, which corresponds to FIG. 17B except that thelight intensity is eight times as high. It has previously been notedthat the cross-hatched regions, when all of the micromirrors are intheir off positions, are provided so that variations in the lightintensity can be absorbed. In FIG. 17E, the size of the angular segmentsfor displaying the LSB+5 bits has been selected so that these bits canbe fully displayed using (for example) four-fifths of each coloredfilter when each of the lamps 378 and 380 is shining at its designbrightness. This leaves one-fifth of each colored filter (i.e., thecross-hatched buffer regions in FIG. 17E) to absorb variations if theintensity of either or both lamps falls to its lowest acceptable levelas a result of aging, etc.

The most significant bits (LSB+6) for all three colors are displayed inthe sixth and seventh revolutions (step 482), as shown in FIGS. 17F and17G. The next frame is then stored (step 484), and the most significantbits for all three colors are displayed during the eighth and ninthrevolutions of the color wheel 368 (step 486). This is shown in FIGS.17H and 171. The bits LSB+5 for all three colors are then displayedduring the tenth revolution of the color wheel 368 (step 486), as shownin FIG. 17J. Thereafter, the bits LSB+4 and LSB+3 are displayed duringthe eleventh revolution (step 488), as shown in FIG. 17K.

The high-intensity lamp 112 is turned off in step 350, and the DMD 346displays all zeros (step 492) during the twelfth revolution (FIG. 17L)as the light level falls to one-eighth of its previous value in thetransition region 458 (FIG. 15). With only the low-intensity lamp 378on, the bits LSB+2 for all three colors are displayed in step 494 duringthe thirteenth revolution (FIG. 17M), and the bits for the LSB and LSB+1for all three colors are displayed during the fourteenth revolution(step 496; FIG. 17N). At this point, the program returns to step 468 tostore the next frame.

From the foregoing, it will be apparent that, in this embodiment, thevideo words for the red, green, and blue components for an image frameare not all displayed during a single revolution of the color wheel 368.Instead, the bits of the video words are displayed during a sequence ofrevolutions and, moreover, more than one revolution is devoted todisplaying the most significant bits. The DMD 346 displays all zerosduring a full revolution of the color wheel during the transition region171 after the high-intensity lamp 112 has been turned on and during thetransition region 187 after it has been turned off. A particularadvantage of this embodiment is that the high-intensity lamp 112 onlyneeds to be turned on and off once every two frames, or 30 times asecond if the frame repetition rate is 60 frames per second.

In the described embodiment, the DMD 46 displays all zeros for a fullrevolution of the color wheel during transition region 440, as shown inFIG. 17C, and for a full revolution during transition region 458, asshown in FIG. 17L. Depending upon the rise time and fall time of lamp380, full revolutions may not be needed. For example, if the intensityof lamp 380 falls very rapidly, FIG. 17L could be omitted altogether.With a fairly rapid descent, it might be necessary to display all zerosonly during the red filter, but it would then be necessary to complicatethe program by starting up again after the all-zeros sector with thegreen filter for the LSB+2 bits, followed by the blue and red filtersfor the LSB+2 bits. Similar comments apply with respect to FIG. 17C andthe transition region 440, with the added observation that it would bepossible to display all zeros for more than one revolution if the risetime of the lamp 380 selected is sufficiently long or turbulent towarrant this.

The details of step 470 are illustrated in FIG. 18A. The red memory 340(FIG. 12) is selected in step 496, and the least significant bits of thered video words stored in memory 340 are read into DMD 346 in step 298.At step 500, a check of the angle information is made to determinewhether the color wheel 368 is positioned at the beginning of the redfilter 386R. If so, the light-level integration value for the leastsignificant bit is loaded into the light-level register 402 during step502, and the least significant bits that were read into the DMD 346during step 298 are latched at step 506. Integrator 398 is reset to zeroduring step 508 and begins integrating toward the light-levelintegration value that was loaded in step 502. In step 510, thenext-to-least significant bits (LSB+1) of the video words stored in theselected memory are read into DMD 346. When the integration value thatwas loaded in step 502 is reached (step 512), the light-levelintegration value for the LSB+1 bits is loaded into register 402 (step514). The LSB+1 bits that were read into the DMD 346 at step 510 arethen latched into the DMD during step 516, so that the DMD stopsdisplaying the LSB bits from the selected memory, and begins displayingthe LSB+1 bits. Integrator 398 is reset during step 518, and beginsintegrating toward the integration value that was loaded intolight-level register 402 during step 514. Then, during step 52, allzeros are read into DMD 340, while the micromirrors of the DMD 46 remainlatched in accordance with the LSB+1 bits. When the integration value isreached, step 522, the zeros that were read into the DMD at step 520 arelatched in step 524. The DMD thus starts displaying one of the hatchedbuffer regions in FIG. 17A.

A check is made at step 526 to determine whether the memory 342, whichstores the video words for the green component of the image, has alreadybeen selected. If not, the green memory 342 is selected during step 528,and the process returns to step 498 to read the least significant bitsof the green component into DMD 346. If the memory 342 has already beenselected, a check is made at step 530 to determine whether the memory344, which stores the video words for the blue component, has alreadybeen selected. If not, it is selected in step 532. If the blue memoryhas already been selected, the process continues to step 472 (FIG. 16)to display the LSB+2 bits of the three colors.

The details of step 474 are shown in FIG. 18B. At step 534, the memory340, which stores the video words for the red component, is selected.The LSB+2 bits of the video words in the selected memory are read intoDMD 346 during step 536, and a check is made a step 538 to determinewhether the color wheel 368 is positioned at the start of the filter forthe selected color. The control unit 338 loads the light-levelintegration value for the LSB+2 bits into light-level register 402during step 540, and the LSB+2 bits are latched into DMD 346 during step542. This begins the display of the LSB+2 bits of the selected color.The integrator 398 is immediately reset to zero during step 544, andbegins integrating toward the light-level integration value that wasloaded during step 540. All zeros are read into DMD 346 during step 548while the DMD continues displaying the LSB+2 bits that were latched instep 542. After the integration value is reached during step 550,however, the zeros are latched into the DMD in step 552, resulting inone of the cross-hatched buffer regions shown in FIG. 17B. A check ismade at step 554 to determine whether the memory 342, which stores thevideo words for the green component, has already been selected, and, ifnot, it is selected during step 556. With the DMD continuing to displayall zeros, the LSB+2 bits for the green component are read into the DMDduring step 536, the position of the color wheel 368 is checked duringstep 538 to determine whether the beginning of the green filter 386G hasbeen reached, and, if so, the integration value for the LSB+2 bits isloaded in step 540. The LSB+2 bits are then latched into the DMD in step542, whereupon the DMD stops displaying all zeros and begins displayingthe LSB+2 bits of the green component.

If the memory 342 for the green component has already been selected whenthe check at step 554 is conducted, a further check is conducted at step558 to determine whether the memory 344 for the blue component has alsoalready been selected. If not, it is selected during step 560 and theprocess returns to step 536.

The details of step 482 (FIG. 16) will now be described with referenceto FIG. 18C. The memory 340 which stores the video words for the redcomponent is selected in step 562. The most significant bits of thevideo words in the selected memory are read into DMD 346 in step 564,and then a check is conducted at step 566 to see whether the color wheel368 is positioned at the beginning of the filter 386 r for the selectedcolor. Since the light-level integration value for the MSB is too largeto be reached during a 120° rotation of the color wheel 368, the controlunit 338 loads half of the integration value into light-level register402 during step 568. The most significant bits are then latched into DMD346 during step 570, thus beginning their actual display. The integrator398 is immediately reset to zero during step 572, and begins integratingtoward the value loaded in step 568. Zeros are read into all locationsof the DMD 346 during step 574 and, after the integration value loadedat step 568 (that is, one-half the light-level integration value for theMSB) has been reached, step 576, the zeros read in at step 574 arelatched into the DMD at step 578, thereby turning all of the pixels off.This corresponds to one of the cross-hatched buffer regions in FIG. 17F.A check is made at step 580 to determine whether the memory 342 for thegreen component has been selected, and, if not, it is selected at step582 and the process returns to step 564. If the green memory 342 hasalready been selected, however, a check is made at step 584 to determinewhether the memory 344, which stores the video words for the bluecomponent, has also already been selected. If not, it is selected atstep 586, and the process returns to step 564. If the blue memory 344has already been selected, steps 582–586 are repeated during the nextrevolution of the color wheel 368 in order to complete the display ofthe most significant bits of the red, green, and blue components.

The Ninth Embodiment

An advantage of the seventh and eighth embodiments is that the lightlevel when the higher-order bits of the video words are displayed isrelatively high, so that the higher-order bits can be displayed in areasonably short period of time. When the lower-order bits aredisplayed, the light level is relatively low, so that these bits neednot be displayed at a speed that unduly taxes the circuitry. Using areduced light level when the lower-order bits are displayed means thatmore time is available for reading them into the DMD than would be thecase if all of the bit ranks were displayed at the same light level. Inthe sixth and seventh embodiments, different light levels were attainedby using a lamp unit 374 having a low-intensity lamp 378 that waspermanently illuminated and a high-intensity lamp 380 that was turned onwhen the higher-order bits were displayed. Another way of achievingdifferent light levels would be to use a single lamp, which iscontrolled so as to emit different light levels as needed. Thispossibility will be discussed in more detail later with reference toFIG. 14.

The ninth embodiment, however, achieves different light levels withoutmultiple lamps and without a lamp that is driven at different emissionlevels. In the eighth embodiment, the lamp unit 374 in FIG. 12 isreplaced by a single lamp (not illustrated) having a constant lightoutput, and lamp-driver unit 366 and intensity register 402 areunnecessary.

FIG. 19 illustrates a color wheel 588 having a frame 590 which mounts ared-color filter 592R, a green-color filter 592G, and a blue-colorfilter 592B. The initial portion of each of these filters has alight-attenuating region 594 which reduces the intensity of the lightemitted by the lamp. As a result, when the color wheel 588 is positionedat the initial portion of any of the filters, the signal from sensor 392in FIG. 12 is reduced and consequently it takes longer for integrator398 to integrate to the light-level integration value stored in register402. This lengthens the time available for displaying the lower-orderbits, and thus also the time available for reading the lower-order bitsinto the DMD.

In FIG. 19, the attenuation regions 594 are integrated with the colorfilters in a single color wheel 588, but if desired, an attenuationfilter wheel that is separate from the color wheel could be used.

Moreover, in lieu of attenuation regions either on the color wheel or aseparate wheel, a ferroelectric LCD could be used to selectively controlthe level of light emitted by a single, constant-output lamp (or aplurality of lamps which together produce a constant output). Onepossibility would be to use an LCD having rows that are all on duringdisplay of the MSB, with half of the rows being on during display of thenext-to-most significant bit, a fourth of the rows being on duringdisplay of the next bit, and so forth. Another possibility would be touse a single ferroelectric liquid crystal cell which is pulse-widthmodulated to provide binary attenuation levels.

The Tenth Embodiment

FIG. 23 illustrates a lighting unit 358′ that is modified with respectto the lighting unit 358 in FIG. 12. Like lighting unit 358, lightingunit 358′ includes a monitor unit 360. However, illumination unit 362′,intensity register 364′, and lamp driver unit 366′ differ from thecorresponding elements of lighting unit 358.

The illumination unit 362′ is different in that its lamp unit 374′consists of a single lamp. It is driven at different binary levels by alamp driver unit 366′ in accordance with a multi-bit light-intensitycommand that is received by intensity register 364′ via a bus 390′. Thelight-intensity command may designate two levels, a low level and a highlevel with eight times the intensity of the low level, as in the firstembodiment. In such a situation, the light-intensity command for the lowlevel would be 0001 and the light-intensity command for the high levelwould be 1000. Alternatively, the light-intensity command may designatea number of different binary light intensities. One possibility would bea straight progression (0 . . . 01, 0 . . . 10, 0 . . . 11, 1 . . . 11),in which case every bit rank of the video words would have its ownintensity. Another possibility would be to use the same light intensityfor pairs of bits in the video words. In accordance with thispossibility, the light-intensity command would be 0 . . . 01 for boththe least significant bit and LSB+1 of the video words, with theexposure being longer for LSB+1. For LSB+2 and LSB+3, thelight-intensity command would be jumped to 0 . . . 10, with the exposurebeing longer for LSB+3 than for LSB+2. Thereafter, the light-intensitycommand would be jumped again, and so forth. It will be apparent thatthe same light-intensity command could also be used for triplets of bitsin the video words, etcetera. Using the same light-intensity command forpairs, triplets, etc. of the video words may be desirable if the lampthat is used requires a relatively long period for stabilization whenthe light intensity is changed.

Instead of using a lamp unit 374′ with a single lamp, the lamp unitcould have two or more lamps that are driven in unison at energy levelsthat change during different time periods. One example would be a lampunit with two lamps that are connected in parallel, in lieu of thesingle lamp shown in FIG. 23.

The Eleventh Embodiment

The prior embodiments have been directed to arrangements in which all ofthe displayed pixels are updated simultaneously, by reading bit valuesinto a DMD while the micromirrors are latched with a bias voltage and bythen momentarily removing the bias voltage so that the micromirrors canrespond to electrostatic forces corresponding the new bit values andmove to their new positions. The present invention, however, is notlimited to displays which can be updated simultaneously; instead, in thepresent embodiment, the bits that are to be displayed are updatedrow-by-row. Although the techniques employed in this embodiment areapplicable to DMDs, they will be explained using an example in which theaddressable spatial light modulator is a ferroelectric liquid crystaldisplay panel. Such a panel is comprised of bi-stable pixels or cells,meaning that they are either on or off without intermediate gray levels,and the cells respond very quickly to applied signals.

In FIG. 20, an input unit 596 has an input terminal 598 for receiving adigitized signal for the red component of an image, an input terminal600 for receiving a digitized signal for the green component, an inputterminal 602 for receiving a digitized signal for the blue component,and an input terminal 604 for receiving a synchronization signal. Thedigitized signals for the red, green, and blue components consist ofseven-bit video data words, so that each video word specifies one of 128levels of red, green, or blue intensity for a point that is to bedisplayed. The video words for the red, green, and blue components arestored in respective frame memories 606, 608, and 610 under the controlof a control unit 612. When a full frame is stored, control unit 612transfers the contents of memories 606–610 to further frame memories614, 616, and 618, and then begins storing the next frame in memories606–610. Control unit 612 also reads out the contents of memories614–618 to an LCD driver unit 620, which addresses a ferroelectric LCDpanel 622 with data from memories 614–618.

The ferroelectric LCD panel 622 has row electrodes and column electrodeswhich cross, with liquid crystal material between them, to provide amatrix of pixels having rows and columns. The row electrodes include afirst row electrode 624, a second row electrode 626, and so on, to alast row electrode 628. The column electrodes include a first columnelectrode 630, a second column electrode 632, and so on, until the lastcolumn electrode 634.

LCD driving unit 620 includes a shift register 636 having the samenumber of stages as there are column electrodes in LCD panel 622. Thefirst stage is connected to an electrically controlled switch 638, thesecond stage is connected to an electrically controlled switch 640, andso on until the last stage, which is connected to an electricallycontrolled switch 642. A switch is closed if its corresponding shiftregister stage contains a one, and it is open if the corresponding stagecontains a zero. All of the switches are connected to a line 644.Driving unit 620 also includes an OFF voltage source 646 which can beconnected by an electrically controlled switch 648 to the line 644, andan ON voltage source 650 which can be connected by an electricallycontrolled switch 652 to the line 644. An inverter 654 is connected to aline 656 from the control unit 612. When line 656 carries a zero, switch652 is open and switch 648 is closed. On the other hand, when line 656carries a one, switch 652 is closed and switch 648 is open. Thus, thesignal on line 656 controls whether OFF source 346 or ON source 350 isconnected to line 644.

The LCD driving unit 620 also includes a row selector 658. It has stageswhich can be strobed to sequentially close an electrically controlledswitch 660 that is connected to first row electrode 624, an electricallycontrolled switch 662 that is connected to the second row electrode 626,and so on to a switch 644 that is connected to the last row electrode628. Each of the switches, when closed, connects the corresponding rowelectrode to ground. When the switches are open, the row electrodes areleft electrically floating.

FIG. 20 also illustrates a lighting unit 666 which includes a monitorunit 668, an intensity register 670, a lamp driver unit 672, a colorselector 674, and an illumination unit 676. Physically, the illuminationunit 676 is disposed behind LCD panel 622, with a light diffusion plate(not illustrated) being inserted between the illumination unit 676 andthe LCD panel 622 in order to spread light emitted by the illuminationunit 676 evenly on the back of LCD panel 622. The illumination unitincludes red fluorescent lamps 678, green fluorescent lamps 680, andblue fluorescent lamps 682. Although only two lamps for each color areillustrated, more may be included if this is desirable to provide evenillumination of the back of LCD panel 622 for each color.

The monitor unit 668 includes a sensor 684 which is positioned to sensethe light emitted by illumination unit 676, an amplifier 686 whichamplifies the signal generated by sensor 684, an analog-to-digitalconverter 688 which converts the amplified sensor signal to a digitalvalue, an integrator 690 which repeatedly adds the digital signal inorder to integrate it, a light-level register 692, and a comparator 694which compares the output of register 694 with the output of integrator690.

The control unit 612 emits a one-bit light-intensity command on line 696to the light intensity register 670. When the light-intensity bit iszero, this indicates that driver 672 is to drive illumination unit 676so that it emits a low-light level. When the light intensity bit ishigh, illumination unit 676 is driven to emit a high-intensity levelhaving a magnitude that is eight times the low-intensity level. Atwo-bit color selection signal emitted by control unit 612 on bus 698indicates which color light should be selected by selector 674. When thecolor selection signal is 00, selector 674 connects driver 672 to thered lamps 678. When the color selection signal is 01, selector 674connects driver 672 to the green lamps 680. When the color selectionsignal is 10, the blue lamps 682 are selected.

Control unit 612 emits a multi-bit light-level integration signal tolight-level register 692 via a bus 700. Register 692 supplies thelight-level integration signal to the comparator 694, whose output tocontrol unit 612 on line 702 is zero as long as the-integrated valuefrom integrator 690 is smaller than the light-level integration signal.When the integrated value reaches the value of the light-levelintegration signal, comparator 694 supplies a one on line 702 to signalcontrol unit 612.

Before describing the operation of the arrangement shown in FIG. 20, itwould be useful to explain how ferroelectric LCD panel 622, with itsbi-stable (on or off) liquid crystal cells, can be used to achieve agray scale. The explanation will be provided by way of analogy to a roomhaving a window with Venetian blinds, the blinds having 60 slats thatcan be opened or closed. Typically, the slats of Venetian blinds arelinked so that they are all opened or closed together, but in thefollowing discussion, it will be assumed that the slats can be opened orclosed individually.

Suppose that it is noon on a cloudless day, so that the illuminationoutside the room is constant and does not fluctuate, and that all 60 ofthe slats are initially closed so that no light enters through thewindow. If we open the top slat (slat number 0), light begins streamingthrough. After a predetermined time delay period, we open the next slat(slat number 1) and light begins streaming through it, too. After twotimes the predetermined delay period, we open the next slat (number 2),and so on, until the bottom slat (number 59) is opened. By the time thebottom slat has been opened, light has been streaming through the topslat for a period of time that is equal to the predetermined delayperiod times 59. Light has been streaming through the next-to-top slat(slat number 1) for a period of time equal to the predetermined delaytimes 58, and so forth. One delay period after the bottom slat has beenopened, we close the top slat; the total amount of light passing throughthe top slat while it was opened is thus proportional to 60 slats timesthe delay period. After another delay period, we close the next-to-topslat; the total amount of light passing through it while it was open isalso proportional to 60 times the delay period. The slats are thusclosed in sequence in this way, and by the time the bottom slat isclosed, the total amount of light that passed through it will again beproportional to 60 times the delay period.

It should be noted that it is not necessary to start the slat-closingsequence immediately after the slat-opening sequence has been completed.When all the slats are opened, the light through each of them is thesame. All that is necessary for a constant amount of light through eachof the slats when the outside illumination does not fluctuate is thatthey are opened in sequence at some particular speed and later closed insequence at the same speed.

Now, consider the case in which the outside illumination level is notconstant, but fluctuates instead. Suppose we are back in our room withthe Venetian blinds at dawn, as the sun is rising and the external lightlevel is thus increasing. If we were to open the slats from top tobottom and then close them from top to bottom at the same speed, theresult would be more light through the bottom slat than the top slat.The reason is that it would grow brighter outside during the timebetween the top slat being opened and the bottom slat was opened, and itwould also grow brighter outside during the time between the top slatbeing closed and the bottom slat being closed. But suppose that, whenthe top slat is opened, we begin integrating the light that passesthrough it. When the integrated light reaches a predetermined value,which will be called an “integration increment Δ,” we open the secondslat. Light is now streaming through both the first slat and the secondslat at the same rate. By the time the integrated amount of lightthrough the first slat has reached two times the predeterminedintegration increment Δ, the integrated amount of light through thesecond slat will reach one times Δ, and we open the third slat. Thisopening process continues to the bottom slat, with the time delaybetween one slat and the next growing shorter because the lightintensity outside is increasing. By the time the bottom slat (number 60)is opened, however, the total amount of light that has entered the roomvia the top slat is proportional to 59 times the integration incrementΔ. If we now begin closing the slats in sequence from the top to thebottom, in accordance with the integrated amount of light, the amount oflight that entered through each slat will be the same as the amount thatentered through every other slat. Furthermore, instead of starting theclosing sequence immediately after the opening sequence has beencompleted, we can allow light to enter through all of the slats for anyamount of time that is needed, and then sequentially close them inaccordance with the integrated light value and still wind up with aconstant amount of light through each of the slats while they were open.

Enough of Venetian blinds. It is time to return to the arrangement shownin FIG. 20. An overview of the operation of this arrangement will now bepresented, followed by a more detailed discussion.

Assume that an old frame has just been displayed and all of the cells orpixels of LCD panel 622 are off. Also assume that the red lamps 678 havebeen selected and are being driven at the low level. Control unit 612emits a one on line 656, thus closing switch 652 and connecting ONsource 650 to line 644. Control unit 612 also reads out a row's worth ofthe least significant bits (LSB) of the red component of the new framefrom memory 614 to shift register 636. Depending on the contents of therow, switches 638–642 may open and close as the row is being shiftedinto register 636, but this has no influence since all of the rowelectrodes 624–628 are floating. After the row has been completelyshifted in, the switches 638–642 have states corresponding to the valuesof the least significant bits of the first row of the red component.Control unit 612 then causes row selector 658 to strobe the first rowswitch 660, thereby connecting the first row electrode 624 to ground. Atthis point, cells in the top row of LCD panel 622 will be turned on byON source 650 if the corresponding column switches 638–642 are closed.Row electrodes whose column switches are open are not connected to ONsource 650, and thus the corresponding cells of the top row of LCD panel622 remain off.

When control unit 612 causes row selector 658 to strobe the first rowswitch 660, thereby causing the least significant bits of the redcomponent for the top row to be displayed on LCD panel 622, it alsoclears integrator 690 to zero and emits a light-level integration valueto register 692. The light-level integration value that is loaded intoregister 692 when the first row switch 660 is strobed (which can becalled “row switch number zero,” corresponding to row number zero of LCDpanel 622) is one times a predetermined integration increment Δ.Integrator 690 then begins integrating toward the light-levelintegration value (1×Δ) stored in register 692. The second row of leastsignificant bits for the red component is then shifted into register636, and when the integrated value from integrator 690 reaches thelight-level integration value, comparator 694 emits a signal on line 702to the control unit 612, which thereupon causes row selector 658 tostrobe the second row switch 662 (row switch number one). Cells in thesecond row of LCD panel 622 are thus turned on in accordance with theleast significant bit of the red component. Control unit 612 thenupdates the light-level integration value in register 692 to two timesΔ, shifts the next row of least significant bits of the red componentinto shift register 636, and so forth. Row-by-row, the cells of LCDpanel 622 are thus turned on in accordance with the LSB bits of the redcomponent, with the light-level integration value that is loaded intoregister 692 being increased in increments of Δ.

After the last row electrode 628 has been strobed, control unit 612opens switch 652 and closes switch 648, thus connecting OFF source 646to line 644. Control unit 612 also clears integrator 690 and again loadsone times the integration increment Δ into register 692 as thelight-level integration value. The first row of least significant bitsof the red component is again shifted into shift register 636, and rowselector 658 strobes the first row switch 660. This turns off the cellsin the top row of LCD panel 622 that were previously turned on. Thecells in the top row that were not turned on are left as they were, thatis, off. The least significant bits of the red component for the secondrow are then shifted into register 636, and the second row switch 662 isstrobed when the value in integrator 690 reaches one times Δ. Thisprocedure continues until all of the cells in LCD panel 622 that wereturned on in accordance with the least significant bits of the redcomponent are turned off in accordance with the least significant bitsof the red component. After they have all been turned off, the sameamount of light has gone through each of the cells that were turned onand subsequently turned off.

After the LSB bits of the red component have been displayed in this way,the next-to-least significant bits (LSB+1) of the red component is alsodisplayed in the same manner. The illumination unit 676 is still drivenat the low level. The difference with respect to the least significantbits is that, after the liquid crystal cells have been turned on inaccordance with the LSB+1 bits, they remain on for a “dwell period” thatis determined by a light-level integration value that is loaded intoregister 692 after the last row has been strobed, and then they areturned off in sequence. For LSB+1, the dwell period is set so that thesame amount of light passes through the turned-on cells as passesthrough during the turn-on and turn-off sequences.

The next-least-significant bits of the red component, LSB+2, aredisplayed in the same manner, with the illumination unit 676 still beingdriven at the low level. The dwell period is three times larger than thedwell period for LSB+1.

After LSB+2 of the red component has been displayed by turning the cellsof LCD panel 622 on row-by-row in accordance with LSB+2 and then turningthem off row-by-row, control unit 612 emits a one over line 696 tointensity register 670. Driver 672 thereupon begins driving illuminationunit 676 at the high level, which is eight times the low level in thisexample. The cells of LCD panel 622 are then turned on and off inaccordance with LSB+3 of the red component. Since the light intensity isnow eight times that when the least significant bits were displayed, thedwell period disappears. This is shown in FIG. 21, where upward arrowsindicate turn-on periods, downward arrows indicate turn-off periods, andhorizontal arrows indicate dwell periods. After LSB+3 has beendisplayed, LSB+4, LSB+5, and the most significant bit, MSB, aredisplayed by turning the cells on in accordance with the respective bitrank and then turning them off after appropriate dwell periods.

After all of the bits of the red component have been displayed, thegreen and blue components are then displayed in the same way. Theapparatus is then ready to display the next frame.

FIG. 22A illustrates the display process described above. In step 704,control unit 612 stores the red, green, and blue components for the nextframe in memories 614–618. It then selects red memory 614 in step 706 tosupply video data to shift register 636.

In step 708, control unit 612 emits a zero on line 692 to intensityregister 670, indicating that driver 672 is to drive illumination unit676 at the low level. A bit rank counter (not shown) within control unit612 is then set to zero, indicating the least significant bit, in step712. The least significant bits of the red component are then displayedon LCD panel 622 in step 714. This will be described in more detaillater.

The bit rank counter in control unit 612 is then incremented in step716. The content of the bit rank counter is then checked, in step 718,to see whether it is greater than two. If not, the process returns tostep 714, and the new bit rank of the red component is displayed. If itis determined at step 718 that the content of the bit rank counter isindeed greater than two, control unit 612 emits a one to intensityregister 670. In response, driver 672 drives illumination unit 676 atthe high level, eight times greater than the low level (step 720). Thedata for the bit rank is then displayed in step 722, and the bit rankcounter is incremented in step 724. Since the most significant bit inthis example is equivalent to LSB+6, in step 726 a check is made todetermine whether the content of the bit rank counter is now seven. Ifnot, the process returns to step 722 for display of the new bit rank.

When the content of the bit rank counter reaches seven (Y at step 726),a check is made at step 728 to determine whether green memory 616 hasalready been selected. If not, it is selected in step 730 in lieu of thered memory 614, and the process returns to step 708. If the green memoryhas already been selected (Y at step 728), a check is made at step 732to determine whether the blue memory 618 has also been selected. If not,it is selected in step 734, and the process returns to step 706. If theblue memory has indeed already been selected (Y at step 732), theprocess returns to step 704 for storage of the next frame.

Step 714 for displaying the data of the bit rank is shown in more detailin FIG. 22B. In this Figure, ON source 650 is selected in step 736 byclosing switch 652. A row counter (not illustrated) in control unit 612is set to zero, meaning the first or top row of LCD panel 622, in step738. Control unit 612 clears integrator 690 to zero in step 740. Datafrom the bit rank of the selected memory that is designated by the bitrank counter, and the row of that bit rank that is designated by the rowcounter, is loaded into shift register 636 in step 742. Then controlunit 612 causes row selector 658 to strobe the row switch (660–664) thatis designated by the bit row counter (step 744). Control unit 612 thentransmits a light-level integration value to light-level register 692 instep 746. It determines this integration value by multiplying apredetermined integration increment Δ by the number of the rowdesignated by the row counter plus one. The light-level integrationvalue after the first row (row number zero) has been strobed is thus onetimes the integration increment Δ; after the second row (row number one)has been strobed, it is two times the integration increment Δ, and afterthe last row has been strobed (if LCD panel 622 has N rows, the last onewould be row number N−1), it is NΔ.

In step 748, a check is made to determine whether the measuredintegration value from integrator 690 has reached the light-levelintegration value stored in register 692. After the integration valuehas been reached, a check is made at step 750 to determine whether thecurrent content of the row counter is N. Since the last row of LCD panel622 is designated as row N−1, the decision at step 750 will be no unlessthe last row of data has already been displayed. If the last row has notbeen displayed, the row counter is incremented at step 752 and theprogram returns to step 742.

If the content of the row counter has reached N at step 750, integrator690 is cleared to zero in step 754. A delay period that is appropriatefor the bit rank designated by the bit rank counter then follows in step756. When the designated bit rank is zero, meaning the least significantbits, the delay during step 756 is zero, as indicated by FIG. 21. FromFIG. 21, it will be apparent that the turn-on period (upward arrow),together with the turn-off period (downward arrow) for the leastsignificant bits permit passage of the smallest quantized value of lightthrough the LCD panel 622, as is appropriate for the least significantbits. Consider the top row of LCD panel 622; half of the smallestquantized amount passes through the top row during the turn-on period,and the top row is the first to be turned off during the turn-offperiod. The total amount of light provided to the top row during theperiod when it is on is thus equal to the integration increment Δ timesthe number N of rows. This same quantity of light is also provided tothe second row during the period while it is on, to the third row, andso forth. To double the amount of light that was provided to each row ofLCD panel 622 during the period when that row was on, the dwell periodfor LSB+1 must thus be such that each row receives an amount of lightequal to an additional ΔN during the dwell period. Since all of the rowsare on simultaneously during the dwell period, the actual time isapproximately the same as the turn-on period or the turn-off period,unless the light intensity varies considerably.

Thus, when the bit rank is one, the dwell period of step 752 is providedby loading a light-level integration value that is equal to N times theintegration increment Δ into light-level register 692. Similarly, forLSB+2, the total quantized amount of light provided to the rows of LCDpanel 622 while they are on should be equal to four times the totalamount of light that was provided to the rows while they were on duringthe display of the least significant bit. This means that thelight-level integration value loaded into register 690 in step 756 whenthe content of the bit rank counter is 2 is equal to 3ΔN. From FIG. 21,it will be apparent that the dwell period for LSB+3 is zero; the dwellperiod for LSB+4 is provided by loading ΔN into light-level register692; the delay period for LSB+5 is provided by loading 3ΔN into register692; and the delay period for the most significant bit is provided byloading 7ΔN into register 692.

With continuing reference to FIG. 22B, switch 652 (FIG. 20) is opened todisconnect ON source 650 from line 644, and switch 648 is closed toconnect OFF source 646 to line 644. This corresponds to off-step 758.Then the row counter in control unit 612 is set to zero in step 760, andintegrator 690 is cleared in step 762. Then, from the selected bit rankof the selected memory, the row of data designed by the row counter RCis shifted into shift register 636 during step 764. The row that hasjust been loaded is strobed during step 766, and the appropriatelight-level integration value is transferred to light-level register 692during step 768. As was the case during the turn-on sequence, thelight-level integration value is the product of the integrationincrement Δ and the content of the row counter plus 1. When the measuredintegration value provided by integrator 690 reaches the light-levelintegration value stored in register 692 (Y in step 770), a check ismade at step 772 to determine whether the last row of LCD panel 622, rownumber N−1, has already been strobed (in which case the content of therow counter will be RC=N). If not, the row counter is incremented instep 7744, and the process returns to step 764. If the content of therow counter is N, however, integrator 690 is cleared at step 776, andthe process then proceeds to step 716 (FIG. 22A).

Returning now to FIG. 20, the ON source 650 and the OFF source 646 maysimply be DC sources, which provide voltages of opposite polarity, callthem “V-ON” and “V-OFF,” that are sufficient for turning the liquidcrystal cells on and off. A cell that is turned on by connecting itmomentarily between ground and V-ON is later turned off by connecting itmomentarily between ground and V-OFF. Since V-ON and V-OFF have oppositepolarities, the cell is not subjected to long-term exposure to the samepolarity, which would be injurious to the LCD panel.

The illumination unit 676 in FIG. 20 includes a plurality of fluorescentlamps for each primary color, the different colors being selected insequence and the lamps for that color being driven at the sameintensity. The intensity is controlled to change between a low level anda high level that is eight times larger. One way that lamp driver 672can accomplish this is by controlling the duty cycle of the lamps of theselected color. For example, driver 672 would supply pulse-widthmodulated energy with a long pulse length for the high-level lightoutput, and pulse-width modulated energy with a short pulse length forthe low-level light output. In contrast to the illumination unit 676 ofFIG. 20, the illumination unit 362 of FIG. 12 includes one lamp thatinherently emits a low level of light and another lamp that can beturned on so that, together, the two lamps emit the high level of light.

In both FIGS. 12 and 21, the illumination units emit light at a lowlevel or at a high level that is eight times larger than the low level.Additional levels could be added. For example, a low level, anintermediate level that is four times greater than the low level, and ahigh level that is sixteen times greater than the low level. It may beinconvenient to do this using lamps that inherently have differentoutput levels, as in FIG. 12. However, in the arrangement of FIG. 21, itwill be apparent that the light-intensity command delivered to register670 could have more than one bit, and the light-intensity valuespecified by the command could be a binary value designated by thesebits.

Another difference between FIGS. 12 and 21 is that color wheel 368 inFIG. 12 provides the sequence of colors, while the lamps with differentcolors are used in FIG. 21. It will be apparent that a color wheel couldbe used with white light to back-light the LCD panel 622 of FIG. 21, orlamps with different colors could be used to illuminate the DMD 346 ofFIG. 12.

The Twelfth Embodiment

In the eleventh embodiment, the pixels of a spatial light modulator wereturned on in accordance with the values of a particular bit rank of thevideo words, and were then turned off in accordance with the same valuesbefore proceeding to a different bit rank. This is useful for an LCDpanel, since it protects against degradation of the panel by ensuringthat the average voltage across the LCD cells is zero. However, it willbe apparent to those skilled in the art that turning the pixels off inaccordance with the same values that were used to turn them on, beforeproceeding to a different bit rank, is not a necessary condition forachieving a constant amount of light through each of the rows of pixels(or, rather, the pixels that are turned on in the rows).

The twelfth embodiment is the same as the eleventh embodiment, exceptthat the pixels are not turned on and then off in accordance with thevalues of one bit rank and then turned on and then off in accordancewith a different bit rank (here, a “different bit rank” will beunderstood to include any bit rank of the video words for the nextframe). In the twelfth embodiment, instead of turning the pixels offrow-by-row before proceeding to a different bit rank, the pixels aresimply adjusted (on or off) on a row-by-row basis in accordance with thenext bit rank. The net result is still a constant amount of lightthrough the pixels that are turned on, for a particular bit rank of thevideo words for a frame, regardless of the row in which the pixels aredisposed.

It will be understood that the above description of the presentinvention is susceptible to various modifications, changes, andadaptations, and the same are intended to be comprehended within themeaning and range of equivalents of the appended claims.

1. A method for displaying an image described by video words of a frame,the video words having bits with different bit ranks, said methodcomprising the steps of: (a) for each bit rank, turning pixels of adigital micromirror device on or off in accordance with values of thevideo words for the respective bit rank; and (b) discontinuouslyexposing the digital micromirror device to brief-duration flashes oflight, the flashes having intensities that depend on the respective bitrank, wherein step (b) comprises exposing the digital micromirror deviceto flashes impinging on the digital micromirror device from a firstdirection, and also to flashes impinging on the digital micromirrordevice from a second direction, and also to flashes impinging on thedigital micromirror device from a third direction.
 2. The method ofclaim 1, wherein some of the flashes are emitted from a red lightsource, others of the flashes are emitted from a green light source, andstill others of the flashes are emitted from a blue light source.
 3. Amethod for displaying a first color component of an image described byvideo words for the first color component of a frame, the video wordshaving bits with different bit ranks, said method comprising the stepsof: (a) for each bit rank of the first color component of the frame,turning pixels of a spatial light modulator on or off in accordance withvalues of the video words of the first color component for therespective bit rank; (b) steadily exposing the spatial light modulatorto light of the first color component during substantially the entiretime that step (a) is conducted, the light being generated by a lightsource; and (c) driving the light source at a first energy level for oneof the bit ranks and at a substantially greater second energy level foranother of the bit ranks, wherein the light source comprises a pluralityof light emitters, and wherein step (c) comprises turning on less thanall of the light emitters to drive the light source at the first energylevel and turning on all of the light emitters to drive the light sourceat the second energy level.
 4. The method of claim 3, wherein thespatial light modulator comprises a liquid crystal display.
 5. Themethod of claim 3, wherein the spatial light modulator comprises adigital micromirror device.
 6. The method of claim 3, wherein step (c)further comprises detecting light emitted by the light source,integrating the detected light, and changing from the first energy levelto the second energy level when the integrated light reaches apredetermined value.
 7. A method for displaying a first color componentof an image described by video words for the first color component of aframe, the video words having bits with different bit ranks, said methodcomprising the steps of: (a) for each bit rank of the first colorcomponent for a plurality of rows of the frame, turning pixels of aspatial light modulator on or off in accordance with values of the videowords for the respective bit rank; and (b) substantially steadilyexposing the spatial light modulator to light that varies substantiallyin intensity while step (a) is conducted, wherein step (b) comprisesshining light generated by a light source onto the spatial lightmodulator, the light source having a plurality of light emitters, andactuating at least one of the light emitters substantially continuouslyas step (a) is being conducted and turning at least one other of thelight emitters on or off as step (a) is being conducted.
 8. The methodof claim 7, wherein the light has an intensity at one moment that is atleast about twice its intensity at another moment.
 9. The method ofclaim 7, wherein the spatial light modulator comprises a liquid crystaldisplay.
 10. The method of claim 7, wherein the spatial light modulatorcomprises a digital micromirror device.
 11. The method of claim 7,wherein the first color component is a red component.
 12. A method fordisplaying a first color component of an image described by video wordsfor the first color component of a frame, the video words having bitswith different bit ranks, said method comprising the steps of: (a) foreach bit rank of the first color component for a plurality of rows ofthe frame, turning pixels of a spatial light modulator on or off inaccordance with values of the video words for the respective bit rank;and (b) substantially steadily exposing the spatial light modulator tolight that varies substantially in intensity while step (a) isconducted, wherein step (b) comprises shining light generated by a lightsource onto the spatial light modulator, detecting the light,integrating the detected light, and changing the intensity of the lightgenerated by the light source when the integrated light reaches apredetermined value.
 13. A method for displaying a first color componentof an image that is generated by a spatial light modulator having pixelsin rows, the first color component of the image being described by videowords for the first color component, said method comprising the stepsof: (a) for a first one of the bit ranks of the video words for thefirst color component, turning pixels in a plurality of the rows of thespatial light modulator on or off in accordance with values of the videowords for the first one of the bit ranks; (b) for a second one of thebit ranks of the video words for the first color component, turningpixels in said plurality of rows of the spatial light modulator on oroff in accordance with values of the video words for the second one ofthe bit ranks; and (c) steadily exposing the spatial light modulator tolight of the first color component while steps (a) and (b) areconducted, the light having an intensity that changes substantiallywhile steps (a) and (b) are being conducted, wherein step (c) comprisesshining light generated by a light source onto the spatial lightmodulator, the light source having a plurality of light emitters, andactuating at least one of the light emitters substantially continuouslyas steps (a) and (b) are being conducted and turning at least one otherof the light emitters on or off as steps (a) and (b) are beingconducted.
 14. The method of claim 13, wherein the spatial lightmodulator comprises a liquid crystal display.
 15. The method of claim13, wherein the spatial light modulator comprises a digital micromirrordevice.
 16. The method of claim 13, wherein the first color component isa red component.
 17. The method of claim 13, wherein step (b) comprisesshining light generated by a light source onto the spatial lightmodulator, detecting the light, integrating the detected light, andchanging the intensity of the light generated by the light source whenthe integrated light reaches a predetermined value.
 18. A method fordisplaying a first color component of an image that is generated by aspatial light modulator having pixels in rows, the first color componentof the image being described by video words for the first colorcomponent, said method comprising the steps of: (a) shining light of thefirst color component on the spatial light modulator, the light beinggenerated by a light source; (b) for a first one of the bit ranks of thevideo words for the first color component, turning pixels in a pluralityof the rows of the spatial light modulator on or off in accordance withvalues of the video words for the first one of the bit ranks; (c)detecting light from the light source; (d) integrating the lightdetected after step (b) is conducted; and (e) for a second one of thebit ranks of the video words for the first color component, turningpixels in said plurality of rows of the spatial light modulator on oroff in accordance with values of the video words for the second one ofthe bit ranks when the integrated light reaches a predetermined value.19. The method of claim 18, further comprising the step of changing theintensity of the light generated by the light source.
 20. The method ofclaim 19, wherein the light source comprises a plurality of lightemitters, and the step of changing the intensity comprises actuating atleast one of the light emitters steadily and turning at least one otherof the light emitters on or off.
 21. A method for displaying a sequenceof image frames described by video words, the video words having bitswith different bit ranks, said method comprising the steps of: (a)exposing a spatial light modulator to light generated by a light source;(b) displaying the bit ranks of the video words describing a given frameof the sequence on the spatial light modulator in a predetermined order;(c) displaying the bit ranks of the video words describing the nextframe of the sequence on the spatial light modulator in a differentorder; and (d) coloring the light to which the spatial light modulatoris exposed with a rotating color wheel, wherein the predetermined orderof step (a) is an ascending order for each color, from least significantbits to most significant bits, and wherein the different order of step(c) is a descending order for each color, from most significant bits toleast significant bits.
 22. The method of claim 21, further comprisingvarying the intensity of the light to which the spatial light modulatoris exposed substantially.
 23. A method for displaying a first colorcomponent of an image described by video words for the first colorcomponent of a frame, the video words having bits with different bitranks, from least significant bits to most significant bits, said methodcomprising the steps of: (a) exposing a spatial light modulator to lightgenerated by a light source; (b) coloring the light to which the spatiallight modulator is exposed with a rotating color wheel, the color wheelhaving at least one segment for the first color component; (c)displaying the least significant bits and the next-least significantbits of the video words for the first color component on the spatiallight modulator during one revolution of the color wheel; and (d)displaying the most significant bits of the video words for the firstcolor component on the spatial light modulator during at least twoadditional revolutions of the color wheel.
 24. A method for displaying acolored image, comprising the steps of: (a) supplying a first set ofdata to a digital micromirror device having an array of pivotablemicromirrors which have pivot axes, the pivot axes of some of themicromirrors being transverse to the pivot axes of others of themicromirrors; (b) exposing the digital micromirror device to light of afirst color that impinges on the digital micromirror device from a firstdirection; (c) supplying a second set of data to the digital micromirrordevice; (d) exposing the digital micromirror device to light of a secondcolor that impinges on the digital micromirror device from a seconddirection; (e) supplying a third set of data to the digital micromirrordevice; and (f) exposing the digital micromirror device to light of athird color that impinges on the digital micromirror device from a thirddirection.